Sforno on Genesis 17:17/ספורנו על בראשית י״ז:י״ז

1?הלבן מאה שנה יולד; He meant that although it is possible for a woman to become pregnant after her prime, when this occurs it is only when she sleeps with a younger man. It has not been known to occur when her partner is also an old man.

2 הבת תשעים שנהת לד, when she sleeps with any man whether young or old. (based on Abarbanel)