John Calvin Commentary Genesis 21:2

"And Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him." — Genesis 21:2 (ASV)

She bare Abraham a son. This is said according to the customary way of speaking, because the woman is neither the head of a family nor gives birth, strictly speaking, for herself, but for her husband.

However, what follows is more noteworthy—that it was in his old age, at the set time that God had predicted—for the old age of Abraham greatly illustrates the glory of the miracle. And now Moses, for the third time, directs us back to the word of God, so that the steadfastness of his truth may always be present to our minds.

And though the time had been predicted both to Abraham and to his wife, yet this honor is specifically attributed to the holy man, because the promise had been especially given for his sake. Both, however, are clearly mentioned in the context.