Bereschit 25:20 Kommentar: Rashi, Rashbam, Sforno & Or HaChaim

בן ארבעים שנה FORTY YEARS OLD — For when Abraham came from Mount Moriah he received the news that Rebecca was born (22:20). Isaac was then thirty-seven years old, because at that time Sarah died and from the birth of Isaac until the Binding — when Sarah died — there were 37 years since she was 90 years old when Isaac was born and 127 when she died, as it is said, (23:1) “And the life of Sarah was [one hundred and twenty seven years]” — thus Isaac was then 37 years old. At that period Rebecca was born and he waited until she was fit for marriage — 3 years — and then married her (Seder Olam).

בן ארבעים שנה, he is described as 60 years old when the twins Esau and Yaakov were born to him by Rivkah. (verse 28) From the combination of these two verses we know that Rivkah had remained barren for 20 years.

אחות לבן הארמי, and from her emerged Esau who was similar in character to the brother of his mother.