Genesis 17:17 ESV states:
Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?
The Bible says that Sarah was 90 years old when she had Issac, which seems remarkable--and by modern standards would be--but Sarah died at the age of 127, meaning 90 was something like...a Biblical 60? And Sarah died young(ish)--Abraham's father was 205 when he died--so really she might have only been 40-something, relatively speaking.
Abraham was 175 when he died, so for him, 100 years old was merely middle aged.
Sarah might be on the cusp of menopause at 90, but why was Abraham doubting his own fertility?
EDIT: I'm aware that Abraham--and even the narrator--thinks that both he and Sarah are old to conceive. I'm trying to reconcile this with all the other evidence that seems to contradict this conclusion.