Rashi on Genesis 17:17/רש"י על בראשית י״ז:י״ז

1ויפל אברהם על פניו ויצחק AND ABRAHAM FELL UPON HIS FACE AND LAUGHED — This word ויצחק Onkelos translates by וחדי which signifies joy (“and he rejoiced’’) but the similar verb in the case of Sarah (ותצחק Genesis 18:12) he translates as meaning laughter. From this you may understand that Abraham had faith and rejoiced, and that Sarah had no faith and sneered, and that is why God was angry with Sarah (when she laughed) but was not angry with Abraham.

2הלבן SHALL A CHILD BE BORN TO ONE WHO IS A HUNDRED YEARS OLD? — There are some questions expressing astonishment which do not imply a doubt but rather affirm the certainty that the circumstance referred to has happened or will happen, as (1 Samuel 2:2) “Did I reveal myself [unto thy father’s house?]” (Of course, I did) and (Ezekiel 8:6) “Seest thou what they do?” (You, of course, have seen it). This question, too, affirms the certainty that it is possible for God to cause Abraham, at the age of hundred, to beget a son and this is in effect what he said to himself: Would there be done to another man one hundred years old this favour that the Holy One, blessed be He, is doing to me”? (Rashi means that the words must be translated, “would a child be born to any other person who is a hundred years old?”).

3ואם שרה הבת תשעים שנה OR SHALL SARAH, WHO IS NINETY YEARS OLD be worthy to bear a son? Although the previous generations used to beget at the age of five hundred years, yet in Abraham’s days the length of life had already become shorter and a diminution of strength had come upon people. Go and learn this from the ten generations from Noah to Abraham who begot their children at the then early age of sixty or seventy.