Genesis 25:19

We have here an account of the birth of Jacob and Esau, the twin sons of Isaac and Rebekah: their entrance into the world was (which is not usual) one of the most considerable parts of their story; nor is much related concerning Isaac but what had reference to his father while he lived and to his sons afterwards. For Isaac seems not to have been a man of action, nor much tried, but to have spent his days in quietness and silence. Now concerning Jacob and Esau we are here told, I. That they were prayed for. Their parents, after they had been long childless, obtained them by prayer, Gen 25:20, Gen 25:21. Isaac was forty years old when he was married; though he was an only son, and the person from whom the promised seed was to come, yet he made no haste to marry. He was sixty years old when his sons were born (Gen 25:26), so that, after he was married, he had no child for twenty years. Note, Though the accomplishment of God's promise is always sure, yet it is often slow, and seems to be crossed and contradicted by Providence, that the faith of believers may be tried, their patience exercised, and mercies long waited for may be the more welcome when they come. While this mercy was delayed, Isaac did not approach to a handmaid's bed, as Abraham had done, and Jacob afterwards; for he loved Rebekah, Gen 24:67. But, 1.

— Matthew Henry's Concise Commentary (public domain)

HISTORY OF ISAAC. (Gen 25:19-34) these are the generations--account of the leading events in his life.

— Jamieson-Fausset-Brown Commentary (public domain)