The Blessing of Esau and Jacob (Genesis 26:34 to Genesis 27:45).
This passage was recorded in writing because it records the blessings given to Jacob and Esau which were in the nature of a binding covenant that could not be changed. They thus testified to the will of Isaac as declared in those blessings. Such a solemn blessing, made with death in view, was often looked on as most sacred and irreversible (compare Deuteronomy 23). That is how Isaac clearly saw it (Genesis 27:33).
‘And when Esau was forty years old he took to wife Judith the daughter of Beeri the Hittite, and Basemath the daughter of Elon the Hittite. And they were a bitterness of spirit to Isaac and Rebekah.'
Esau further demonstrates his contempt for his status when he marries two Hittite women. The tradition of marrying within the family meant little to him, and his acts brought great grief to Isaac and Rebekah. But as the eldest son he would have been expected to marry within the family. In the writer's eyes this introductory sentence is a silent commentary on why Esau loses his firstborn's blessing.
“When Esau was forty years old.” Again a round number indicating full maturity. If we take the numbers literally this would make Isaac about one hundred years old. But Isaac also married at forty. This would suggest that this round number is used to indicate marriageable age.