The Book of Jasher is mentioned in Joshua 10:12–13 and 2 Samuel 1:18. This is interesting – although the Book of Jasher is not part of the canon, the Old Testament does mention it. So, it was in existence. Let’s read Jasher 24:37-45:
37. And the Lord hearkened to the voice of Eliezer, for the sake of his servant Abraham, and he happened to meet with the daughter of Bethuel, the son of Milcah, the wife of Nahor, brother to Abraham, and Eliezer came to her house.
38. And Eliezer related to them all his concerns, and that he was Abraham’s servant, and they greatly rejoiced at him.
39. And they all blessed the Lord who brought this about, and they gave him, Rebecca, the daughter of Bethuel, for a wife for Isaac.
40. And the young woman was of very comely appearance, she was a virgin, and Rebecca was ten years old in those days.
41. And Bethuel and Laban and his children made a feast on that night, and Eliezer and his men came and ate and drank and rejoiced there on that night.
42. And Eliezer rose up in the morning, he and the men that were with him, and he called to the whole household of Bethuel, saying, send me away that I may go to my master; and they rose up and sent away Rebecca and her nurse Deborah, the daughter if UZ and they gave her silver and gold , men servants and maid servants, and they blessed her.
43. And they sent Eliezer away with his men; and the servants took Rebecca, and he went and returned to his master to the land of Canaan.
44. And Isaac took Rebecca and she became his wife, and he brought her into the tent.
45. And Isaac was forty years old when he took Rebecca, the daughter of his uncle Bethuel, for a wife.
What we read from Jasher 24:35-45 is that Isaac was 40 years old when he consummated his marriage with 10 year old Rebekah. Some people might raise their eyebrows at the above information. It is noteworthy to mention to viewers that this was common in human history. This type of marriage was customary among all societies and cultures. Most of the time, the man was always older than the girl. It is only in the 20th century where this started changing. Prior to the 20th century, when a girl hit puberty, she was considered to be a ‘woman’, and it didn’t make a difference if she was 9, 11 or 14. Children were classified into adulthood upon reaching the stage of puberty. There are many historical evidences for this.
The age of consent in America and most European countries was 10, and in the state of Delaware it was as low as 7, only 100 years ago. Other cultures were the same, as Rabbi Isaac Klein (and other scholars) states in his book: ‘A Guide to Jewish Religious Practice’, child marriages were common and that ‘age was not the factor’ when someone was married off,
“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.” 
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…” 
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.” 
Additionally, some scholars disprove the Book of Jasher as not being authentic because there are ‘some’ instances where the information in Jasher contradicts the Old Testament or that it did not exist prior to the 16th century. However, this type of reasoning should not negate the whole Book, for there are many parts which agree with the Old Testament. Mordecai Manuel Noah who translated Jasher had this to say on the book:
Without giving it to the world as a work of Divine inspiration, or assuming the responsibility to say that it is not an inspired book, I have no hesitation in pronouncing it a work of great antiquity and interest, and a work entitled, even regarding it as a literary curiosity, to a great circulation among those who take pleasure in studying the Scriptures. – Mordecai Manuel Noah [New-York, April, 1839] 
Mordecai states although the book may not be ‘divine inspiration’, he continues by saying that he has ‘no hesitation in pronouncing it a work of great antiquity’.
In addition to what I have already presented, the following passage sheds more light and corroborates with the previous evidences shown that Rebekah was a very young girl when she got married. In fact, she was so young that she needed a nursing woman with her when she was married off:
Genesis 24:57 Then they said, “Let’s call the young woman and ask her about it.” 58 So they called Rebekah and asked her, “Will you go with this man?” “I will go,” she said. 59 So they sent their sister Rebekah on her way, along with her nurse and Abraham’s servant and his men. 60 And they blessed Rebekah and said to her.
Question: If Rebekah was a grown woman, who attained full womanhood, why would she need a ‘nursing woman’ to accompany her? This shows she was very young and not an age where she can take care of herself. This above verse confirms Jasher 24:35-45 that Rebecca was most probably 10 years old when she was married off to 40 year old Isaac.
Thus, historically speaking, marriages such as Isaac marrying 10 year old Rebekah when he was 40 years old would be culturally and religiously acceptable in those times. Seeing examples such as age of consent in America and Europe gives more weight that such marriages was not uncommon. In addition, as I stated earlier, age was not a factor when a girl hit ‘puberty’, as before the 20th century she was considered to be an adult. Also, in Genesis 24:59, Rebekah having a nursing woman accompany her proves that she was very young – why would she need a nurse with her if she was a fully grown woman?
Credit Call to the truth
 A Guide to Jewish Religious Practice By Isaac Klein page 396
 Story as Torah: Reading the Old Testament Ethically By Gordon Wenham page 84
 Sabbath and Jubilee (Understanding Biblical Themes) By Dr. Richard H. Lowery page 8
 The Ancient Book of Jasher In a Translation Based on the Original Hebrew With The Original Introductory Notes To The English Edition. The Ancient Book Of Jasher referred to in Joshua And Second Samuel [NEW YORK: Published By M. N. Noah & A. S. Gould, At 144 Nassau-Street – First Edition 1840] page 8