John Calvin Commentary Genesis 21:5

"And Abraham was a hundred years old, when his son Isaac was born unto him." — Genesis 21:5 (ASV)

And Abraham was an hundred years old. Moses again records Abraham's age to better draw his readers' attention to the miracle. And although only Abraham is mentioned, let us still remember that he is here presented to us, not as a man of lust, but as Sarah's husband, who obtained through her a lawful offspring in extreme old age, when the strength of both had failed.

For the power of God was most clearly seen in that, after their marriage had been fruitless for more than sixty years, they suddenly obtained offspring. Sarah, truly, to make amends for the doubt to which she had yielded, now exultingly proclaims God's kindness with fitting praises. And first, she says that God had given her reason for joy—not common joy, but such as would cause everyone to congratulate her. Secondly, to amplify this, she assumes the character of an astonished inquirer: ‘Who would have told this to Abraham?’ Some explain the phrase in question, ‘will laugh at me,’ as if Sarah had said, with shame, that she would become a byword among the people. But the former meaning is more suitable; namely, ‘Whoever hears it, will laugh with me;’—that is, to congratulate me.