Adam Clarke on Genesis 25:21

Isaac entreated the Lord for his wife - Isaac and Rebekah had now lived nineteen years together without having a child; for he was forty years old when he married Rebekah, Gen 25:20, and he was threescore years of age when Jacob and Esau were born, Gen 25:26. Hence it is evident they had lived nineteen years together without having a child.

The form of the original in this place is worthy of notice: Isaac entreated Jehovah, לנכח אשתו lenochach ishto, directly, purposely, especially, for his wife. Ainsworth thinks the words imply their praying together for this thing; and the rabbins say that "Isaac and Rebekah went on purpose to Mount Moriah, where he had been bound, and prayed together there that they might have a son." God was pleased to exercise the faith of Isaac previous to the birth of Jacob, as he had exercised that of Abraham previous to his own birth.