In Biblical times people were married at a very young age. Girls were usually betrothed before they reached puberty – majority of the time the marriage would have consummated when the girl reached puberty, and that was usually between the ages of 8, 9 or older,(Note: when a girl reached puberty prior to the 20th century, she was considered to be an adult in most cultures/societies).
In this article I will mostly quote Scholarly sources to prove that marriage in ancient Israelite times took place at a very young age, sometimes the girls who were married off were pre-pubescent. There was no law against a pre-pubescent girl being married off. Actually as you will read further, you will come to realise that the Mishnah gave approval for a Man to have intercourse to a betrothed girl, any-time after the age of three years old.
Isaac’s wife was Rebecca, mother of Jacob, and Esua. According to calculations made by Rabbi Solomon Itzhaki (A.K.A. Rashi a well-known respected Jewish Scholar), Rebecca was three years old when she married Isaac.
1. Rashi’s commentary on Genesis 25:20 says:
forty years old: For when Abraham came from Mount Moriah, he was informed that Rebecca had been born. Isaac was then thirty-seven years old, for at that time Sarah died, and from the time that Isaac was born until the “Binding” [of Isaac], when Sarah died, were thirty-seven years, for she was ninety years old when Isaac was born, and one hundred and twenty-seven when she died, as it is stated (above 23:1): “The life of Sarah was [a hundred and twenty-seven years.”]
This makes Isaac thirty-seven years old, and at that time, Rebecca was born. He waited for her until she would be fit for marital relations-three years-and then married her.— [From Gen. Rabbah 57:1
[Retrieved it from this website: http://www.chabad.org/library/bible_cdo/aid/8220/showrashi/true]
2. Also Johann Buxtorf cites Rashi that Rebecca was three years old when she married Isaac.
“Rabbi Solomon in his comment on Genesis, says that Rebecca, when she was married to Isaac, was but three Years of Age. His words run thus, ‘When Abraham was come from Mount Moria, he received the joyful News of Rebecca. Isaac was at that Time Thirty seven years old; and then did Sarah die. The time, from birth of Isaac to the death of Sarah, was Thirty seven Years, And Sarah was Ninety Years old when Isaac was born; and One Hundred and Twenty Seven Years old when she died: As it is said in Gen 23:1 . Sarah was one hundred and twenty-seven years old. Behold, the Age of Isaac was Thirty Seven Years, at the Time of the Birth of Rebecca. And when he had waited for her three Years, till she was fit for marriage, he took her to wife.”
According to this Account, Rebecca was a very notable Girl at three years of age. But that a girl of three Years old is fit for marriage, is maintained very plainly in the Jewish writings; particularly, in Emek Hamelech, in the following passage, ‘our blessed sags, of blessed memory, say, that a female is not fit for marriage, ‘till she is arrived at the Age of three years and one day.’ The Talmud supports these Sages here, in the part entitled Avoda Sara. And the Sanhedrin says, A daughter, who is of the age of three years and one day, is, by being bedded with a Man, lawfully married.”
Before going any further, one of the objections raised by some Jewish and most Christian Apologist is that they say, “how can a three year old fetch water out of a well?” They further say, “a girl of that age could not do that she must be a lot older than three.” These statements do not disapprove anything. The way they make it seem, as if it is impossible for a girl to pull 1, 2 letters of water. Any three old girl could pull 1, 2 letters of water out of a well.
3. In addition to what we have already brought forth on Isaac’s marriage to Rebecca, The Zohar, which is Translated/Commentary by Daniel Chanan Matt also makes mention that Isaac married Rebecca when she was three years old.
51. “She was three years old when he embraced her the youngest legal age a female can be-married. See M Niddah 5:4; Seder Olam Rabbah 1; Soferim, add. 1:4 Rashi on Genesis 25:20 Tosafot, Yevamot 61b, s.v. ve-khen; sekhe; Tov, Genesis 24:14.
Rebekah’s three years correspond to the three colors of the rainbow.
52. He engendered at sixty, generating fittingly… see Genesis 25:26. When Isaac fathered Jacob he was sixty years old, symbolizing the full sextet of sefirot from hesed through Yesod, thereby ensuring that Jacob would be complete. 
As you have read it is quite clear, a fact that Rebecca married Isaac when she was three years old. Furthermore, we have more evidence from Judaic Holy Book ‘Mishnah’, it gives more weight to the above references that girls as young as three years old were allowed to be betrothed by intercourse at the age of three.
Although in Ancient Hebrew marriages girls were recommend to be married at the age of 12, there are laws in the Mishnah that give approval that once betrothed you can have sexual intercourse any-time after the age of three years old.
4. Jacob Neusner is an American academic scholar of Judaism. In the Book: ‘The Comparative Hermeneutics of Rabbinic Judaism: Seder Tohorot. Tohorot through Uqsin.’ The Jewish oral Torah i.e. Mishnah states,
M. 5:4 A girl three years and one day old is betrothed by intercourse. And if a Levir has had intercourse with her, he has acquired her. And they are laible on her account because of the law [Prohibiting intercourse with] a married woman. And she imparts uncleanness to him who has intercourse with her [when she is menstruating] to convey uncleanness to the lower as to the upper layer.
[If] she was married to a priest, she eats heave offering. [If] one of those who are unfit [for marriage] has intercourse with her, he has rendered her unfit to marry into priesthood. [If] one of all those who are forbidden in the Torah to have intercourse with her, he is put to death on her account, but she is free of responsibility. 
5. A Commentary on the above verse, in the Book: ‘A history of the Mishnaic Law of Purities. 15. Niddah. by Jacob Neusner, it says:
M. 5:4-5 bring us to the next stage in the matter of the legal status of children, female and male. The girl three years and one day old is deemed capable of sexual relations, which accounts for A, B, and C. D. Presumably should not apply to M. 5:3-a; if the girl is unclean as a menstruant but is incapable of sexual relations, one who has (or attempts) relations with her is not made unclean as is one who has had sexual relations with a menstruant. E simply goes over familiar ground; since the girl can be acquired as a wife, she also may eat heave-offering. F. Follows, and G. Repeats what is already obvious.
But H limits the matter. The girl is not held responsible in a matter of forbidden sexual relations. I is a minor gloss. If the girl is less than three years and one day old, we do not regard the sexual relationship as of legal consequence. The theory is that the tokens of Virginity are restored before that time but not afterword. 
6. The Anglican Priest Herbert Danby in the Book: ‘The Mishnah: Translated from the Hebrew with Introduction and Brief Explanatory notes.’ It says that a girl of three years old and one day could be betrothed by the brother’s husband and he can have intercourse with her.
“A girl three years old and one day may be betrothed by intercourse; her deceased childless husband’s brother can acquire her by intercourse; and by connexion with her a man can be culpable by virtue of the Law of a married woman; and him that has connexion with her [while she is a menstruant] she renders unclean so that he conveys uncleanness to what is beneath him in like degree as [he that has a flux conveys uncleanness] to what lies above him’ if she married to a priest she may eat of Heave-offering;
if one that is ineligible has connexion with her he renders her ineligible for marriage with a priest; if any of the forbidden degrees prescribed in the Law had connexion with her they are put to death on her account, but she is not culpable. If she is younger than this, it is as one that puts a finger in the eye.” 
7. Edward Hendrie also echoes the same statements made previously
“Orthodox Judaism has a very permissive attitude towards sexual deviance. For example:Sanhedrin 55b: It is permitted to have sexual intercourse with a girl three years and one day old. See also Yebamoth 57b, 60b; Abodah Zarah 37a.
Kethoboth 11b: When a grown-up man has intercourse with a little girl it is nothing, for when the girl is less than three years old it is as if one puts the finger into the eye, tears come to the eye again and again, so does virginity come back to the little girl under three years.” 
8. Professor Joshua A. Fogel in his book: ‘Reading Tractate Avodah Zarah of the Babylonian Talmud.’ He says the: ‘tumah of a zav’ is when it happens that is when a boy is ready to cohabit. The Hebrew word ‘zav’ means, In Torah terminology, the Hebrew word zav (lit. “flow”) is a state of ritual impurity arising from abnormal seminal discharge from the male sexual organ” (see Wikipedia.com). Here is what is said,
“The orevious daf ended with R. Yehudah ha-Nasi accepting the view of R. Chiyya that an idolater conveys the tumah of a zav from the nine years and one day (not just one day). When asked about this age, he replies that this is the age at which he is able to cohabit and thus convey the tumah of a zav. If nine seems on the young side, it’s three for girls. In other tractates of the Talmud, these ages, especially for a girl, become extremely relevant, such as in determining what constitutes rape or marriage by cohabitation;
I mentioned in here only because how shockingly young it seems to my (and probably most) 21st century eyes. It is Ravina who argues for age of three for girls, rather than from birth which was put forth by Rav Nachman bar Yitzchack on the previous daf. Ravina contribution is to match age cohabitation with age of zav (or niddah) tumah capacity.” 
9. Robin Grille
“Among the ancient Hebrews, betrothal by sexual intercourse was permitted with girls aged three years and day, and marriages for girls was actually recommended at 12 years of age. The Biblical woman and child were property, not persons. For the right to marry a pre-pubescent girl, one simply had to pay an agreed sum to her father. In India, child marriage was condemned by law until 1929, although around 80 per cent of the population was still practicing it.” 
10. Mary De Young
“The possession of Children by their parents was also given religious sanction in the teachings of both the Talmud and the Bible. Rush (1880) states that the Talmud teaches that a girl of ‘three years and one day’ could be betrothed through an act of sexual intercourse.” 
11. Professor Geza Vermes who is well-known and a respected scholar comments that Pre-pubescent girls were allowed to be married.
“…the Greek parthenos could also mean that the girl was young and/or unmarried. In fact, in the Septuagint translation of the Old Testament parthenos was used to render three distinct Hebrew words, ‘Virgin’, ‘girl’ and ‘young woman’. Already Rabbis in the Tannaitic era (first to second century ad) subscribed to further nuances, and there is no reason to think that all these were invented by them.
Even the word betulah, which normally means virgo intact, when used by them could carry the laternal sense of bodily immaturity with the consequential inability to conceive. In Rabbinic terminology this type of virginity in a woman ceased with the physical onset of puberty.
The Mishnah, the oldest of the rabbinic codes, defines a virgin as a female who ‘has never seen blood even though she is married’ (mNiddah 1:4). The Tosefta, another early Jewish code of law, claims in the name of Rabbi Eliezer ben Hyrcanus (late first century ad) that such a woman would continue to count as a virgin even after she had conceived and borne children without prior menstruation (tNiddah 1:6)!
To understand these statements, we must remember that in the InterTestamental and early rabbinic age, pre-puberty marriage was generally permitted. In fact rabbis seriously debated whether bloodstains found after the wedding night in nuptial bed of a minor, i.e. a ‘virgin in respect of menstruation’, marked her first period or the consummation of the marriage.” 
12. Reverend Kathryn J. Riss also makes mention that in first century parents married off their daughters who were pre-pubescent to much older men. What is interesting is she does not mention once that Rabbis or anyone higher up in authority speaking against such marriages.
“The longest New Testament passage on marriage is found, not in Ephesians, but in 1 Corinthians chapter 7. In stark contrast to the legal positions and social expectations of the first century, here the rights and responsibilities of man and woman are upheld as equal. Although marriages were arranged by parents, who often espoused their pre-pubescent daughters to much OLDER MEN…” 
13. Theological Dictionary of the Old Testament, by G. Johannes Botterweck, Helmer Ringgren and Heinz-Josef Fabry it states,
“One might counter that the expression mohar habbetulot (Ex. 22:16) refers rather to the pretium virginitatis. In this case, the mohar would be compensation to the girl for the loss of her virginity. This explanation, however, is unacceptable, since it proceeds on the assumption that the term betula means ‘virgin.’
This may doubtlessly be the case in many passages, but in joel 1:8, betula thus refers to a married woman who had been ‘possessed’ by her husband (ba’al); betula thus refers to a marriageable girl who was physically able to cope with a man, ‘taking her into his possession.’ Here the term betula says nothing about her virginity. Ex.22:16 (17) (kesep yisqol kemohar hab betulot) can thus be translated ‘he shall weigh out as much silver as is required for marriageable girls.’
In this context we should point out that ancient Hebrew custom did not associate marriageability with puberty. In contrast to the marriageable girl (betula), the…. Alma refers to a girl in puberty capable of conception. Girls could in fact already be given marriage long before actual physical maturity, perhaps even as young as five years old (cf. Lev. 27:5), and it did happen that marriages were already consummated with prepubescent girls. 
14. Zeʼev Wîlhelm Falq
“A more common practice was the marrying off of minor daughters by their fathers. For instance King Agrippa(s) (41-44 C.E.), left three daughters: ‘of these, one, Berenice, who was sixteen years old, was married to Herod, her father’s BROTHER, and two were unmarried, namely Mariamne and Drusilla, aged respectively ten and six years. They had been promised by their fathers in marriage,
Marriamne to Julius Archelaus, son of Helkias, and Drusilla to Epiphanes, the son of Antiochus, King of Commagene’ (Josephus, XIX Antiquites, 9. 1. 354). In such cases, marriage and consummation would be postponed until the BRIDE REACHED PUBERTY: it was held proper to marry off a minor daughter with her consent and not against her will. However, these rules were not only applicable to a minor daughter given in marriage by her father, since his right to do so is expressly mentioned in the Torah (Exodus XXI 7; Deuteronomy XXII 16).” 
15. T. J. Wray
“In the Hebrew Bible and New Testament, marriage is considered the natural state for men and women, and although there are certainly instances where Jewish men (and rarely, Jewish women) are not married, this is the exception rather than norm. Most women marry quite young, usually soon after the onset of menstruation, which of course, heralds fertility. Men typically marry later (in their twenties, or even thirties) and this most men are older than their wives.” 
16. Harry L. Tabony
“BC. The good book of Genesis says that after a few generations Abraham’s progeny had grown to sizeable numbers. That is understandable, considering the practice of multiple wives and marriage just after puberty for girls. Abraham’s many sons could father thousands of children, considering ample girls available from the natives of Canaan.” 
17. Encyclopedia of the Jewish Diaspora: Origins, Experiences, and Culture, Professor Mark Avrum Ehrlich writes:
“Girls were often married to their uncles on their father’s side, or to their cousins, in order to secure the family’s capital and in the hope that their kin would take good care of them. Minor girls were betrothed by their fathers (by Kidushin, a legally binding commitment) even before they came of age, and usually began living with their husbands-occasionally much older than them- at the age of puberty.” 
18. Respected Rabbi Isaac Klein in his book: ‘A Guide to Jewish Religious Practice’, says that Child marriages were common and that ‘age was not the factor’ when someone had to get married. So in other words no matter what the age of the girl, once the parents gave the go ahead, the girl had to choose the husband, she had no choice in the matter.
“Child marriages were very common in ancient days. Since marriages were arranged by parents and the consent of parties was not necessary, AGE WAS NOT THE FACTOR in coming to an agreement. The physical factor related only to the consummation of the marriage. Hence, there was usually a waiting period between the agreement and the consummation.
It is logical to assume that when a boy and a girl reached the age of puberty, and the sex urge demanded satisfaction, ancient society deemed marriage to be the answer. In time, other elements became factor in marriage: climate, social conditions, economic conditions, and even political conditions.” 
19. Old Testament Scholar Gordon Wenham
“In reality social custom and pressures greatly curtailed the sexual freedom of men in the Bible times. First, marriages were customarily arranged by parents, as soon as children passed puberty. This meant that there were few unattached girls: most females of marriageable age were either married or betrothed….” 
20. David M. Carr Professor of Old Testament
“…ancient Israelite women did have to deal with a constant stream of pregnancies, from when they reached puberty in their teens until they died in their twenties or thirties.” 
21. Dr. Richard H. Lowery
“A typical adult Israelite male had a life expectancy of forty years. Infant mortality rates were high, perhaps as high as 50 percent. So women typically had two pregnancies for every one child who reached age five. Since the economic survival of the household depended on the production of able-bodied children, women married immediately after puberty and were pregnant or nursing for a relatively large portion of their life.” 
22. Saba Soomekh
“In the early twentieth-century, Iranian Jewish women married at a very young age, either when they reached puberty or by their fifteenth year or sixteenth birthday. Similarly, Jewish women living in Ottoman Libya were married once a girl reached puberty and under Italian rule, ‘women married when they reached their fifteenth or sixteenth birthday, but at times, the brides were much younger.” 
23. Steven M. Lowenstein Professor of Jewish History
“The next great life cycle event in Jewish life after the bar Mirzvah is marriage. In most traditional societies (Jewish and Non-Jewish), marriages were arranged between families, with only the most perfunctory consultation with the couple to-be, and often involved complex financial arrangements such as dowries and trousseaus. In the middle ages the age at marriage seems to have been around puberty throughout the Jewish world. …” 
24. Ruth Lamdan
“The numerous references to child marriage in the 16th- century Responsa literature and other sources, shows that child marriage was so common, it was virtually the norm. In this context, it is important to remember that in halakha, the term ‘minor’ refers to a girl under twelve years and a day. A girl aged twelve and a half was already considered an adult in all respects.” 
25. Philip J. King
“A few anecdotal incidents, and a wealth of later documentation, suggests that women married young, while still in their teens, sometimes early teens, in fact; men waited until well into their twenties or even early thirties before marrying.” 
26. Ken Blady
“Persian Jews married off their children very early: boys at ten or twelve, and girls at the very tender age of six or seven. Some girls were mothers by age twelve or thirteen. Polygamy was practiced only by men who could afford more than one wife. It was common for a prepubescent girl to marry a man who was old enough to be her grandfather.” 
27. Professor Harvey E. Goldberg
“Habbani Jewish ‘women’ were almost always prepubescent at their first marriage. Their first husbands were often in their late teens. Normally, there was neither bride price nor dowry, but a groom was expected to provide wedding jewelry and seven goats for the wedding festivities. The jewelry became a woman’s property. Occasionally, the bridegroom was far older than the bride.” 
28. Jonathan B. Krasner says that betrothal took place at the age of eight or nine, now keep in mind earlier we provided evidences that once a girl was betrothed the husband can engage with her sexually:
“Betrothal, or engagement, generally occurred at the age eight or nine. Jewish girls typically married at age eleven or twelve and boys at about thirteen or fourteen. (In Germany and France, Christian girls typically married at twelve or thirteen, and boys were usually in their late teens or twenties.)… (Marriage in Ashkenazic).” 
I believe who ever reads this article will agree that Judaism in past practised and allowed pre-pubescent marriages. Also, Isaac’s marriage to Rebecca, all the evidence shown is in agreement that Isaac married Rebecca when she was three years old. Whatever some Christian/Jews (modern) may say about Rebecca being older because she fetched water out of a well, they have no proof that a girl of three cannot fetch 1 – 2 litters of water out of a well. I also gave many references that the Mishnah gave approval for girls to be betrothed by intercourse at the age of three
credit call to the truth
Johann Buxtorf, Johann Andreas Eisenmenger, John Peter Stehelin
Rabinical literature: or, The traditions of the Jews, contained in their Talmud and other mystical writings. Likewise the opinions of that people concerning Messiah, and the time and manner of his appearing; with an appendix comprizing Buxtorf’s account of the religious customs and ceremonies of that nation; also, A preliminary enquiry into the origin, progress, authority, and usefulness of these traditions; wherein the sense of the strange allegories in the Talmud and Jewish authors is explained. [ Publisher: London J. Robinson, 1748] Volume 1 page 33 – 34
 The Zohar: Pritzker Edition. Volume two, (2003) page 264 [The Zohar 1:136b] Translation and Commentary by Daniel Chanan Matt
 The Comparative Hermeneutics of Rabbinic Judaism: Seder Tohorot. Tohorot through Uqsin By Jacob Neusner Volume 6 [Copyright 2000] page 152
 A history of the Mishnaic Law of Purities. 15. Niddah . Commentary edited by Jacob Neusner page 83
 The Mishnah: Translated from the Hebrew with Introduction and Brief Explanatory notes Herbert Danby page 750
 Solving the Mystery of Babylon the Great By Edward Hendrie
 Reading Tractate Avodah Zarah of the Babylonian Talmud By Professor Joshua A. Fogel page 84
Parenting for a Peaceful World By Robin Grille
Mary De Young The sexual victimization of children page 103
 The Changing Faces of Jesus By Geza Vermes
 Journey’s End: Removing Biblical Barriers Between Women and Their Destiny By Reverend Kathryn J. Riss, THM page 164
 Theological Dictionary of the Old Testament, Volume 8 edited by G. Johannes Botterweck, Helmer Ringgren, Heinz-Josef Fabry  page 144 – 145
 Introduction to Jewish law of the second Commonwealth. 2 (1978) By Zeʼev Wîlhelm Falq part 2 page 278
 What the Bible Really Tells Us: The Essential Guide to Biblical Literacy By T. J. Wray page 146 chapter 7
 Christianity’s Source: It All Came from Ancient Egypt By Harry L. Tabony
 Encyclopedia of the Jewish Diaspora: Origins, Experiences, and Culture, Volume 1 By Mark Avrum Ehrlich page 258
 A Guide to Jewish Religious Practice By Isaac Klein page 396
 Story as Torah: Reading the Old Testament Ethically By Gordon Wenham page 84
 The Erotic Word: Sexuality, Spirituality, and the Bible By David M. Carr Professor of Old Testament Union Theological Seminary In New York page 43
Sabbath and Jubilee (Understanding Biblical Themes) By Richard H. Lowery page 8
 Saba Soomekh Between Religion and Culture: Three Generations of Iranian Jewish Women from the Shahs to Los Angeles Page 65
 The Jewish Cultural Tapestry : International Jewish Folk Traditions By Steven M. Lowenstein, Professor of Jewish History, University of Judaism page 108
 A Separate People: Jewish Women in Palestine, Syria, and Egypt in the sixteenth Century By Rûth Lamdān page 47
 Life in Biblical Israel By Philip J. King, Lawrence E. Stage page 37
 Jewish Communities in Exotic Places By Ken Blady Page 69
 Sephardi and Middle Eastern Jewries: History and Culture in the Modern Era edited by Harvey E. Goldberg page 267
 The History of the Jewish People: A Story of Tradition And Change By Jonathan B. Krasner, Jonathan D. Sarnapage volume 1 page 83