And she said, “Who would have said to Abraham that Sarah would nurse children? Yet I have given birth to a son in his old age.”
Genesis 21:7 (NASB)
There is a teaching in the Jewish commentaries that say Sarah experienced a reversal of age and literally became a young woman again in order to give birth to Isaac and to nurse him until weened.
In this chapter, when Isaac is born, only Abraham’s age is referenced.
Whether Sarah literally became a youthful woman again (so beautiful that even a Pharaoh would find her desirable!) or not is immaterial. It changes nothing. However, in some ways, she did experience something that's only possible for younger women – to give birth, and to nurse a child until it is weened.
In contrast, Abraham appears to remain old in every way.