[Leonine]

Good post, but remember that Abraham did have a child by Keturah in his mid-80s. His name was Ishmael.

[Cicero]

Oh, yes. And its possible for a man to father a child at the age of 80. But it's unlikely that a man and woman of a hundred could have a child. Especially since Sarah says to the angel that it has been many years since they have known each other.

A similar things happens at the beginning of the Gospel of Luke, when the coming of John the Baptist is announced to his father. The gospel even echoes the angel's words to Abraham and Sarah, that with God all things are possible.