Why was it incredulous to Abraham and Sarah that God promised them a Son, when it was common to have children in old age, per the narrative in scripture?

Can we anyone offer any opinion to this question?
Terah was 70 when he begat Abraham & Abraham is 75 when God promises him a son.
Why was it incredulous to Abraham and Sarah that God promised them a Son, when it was common to have children in old age, per the narrative in scripture?

Ed Whyff

Post-diluvian humanity was already experiencing the effects of the change in the earth’s ecosphere. Whereas before the Flood people lived hundreds of years, and now after the flood age longevity was shrinking and people could see it. It was hard to tel… See more

W Charles Brown

I thought the focus here was " GOD PROMISED AND DELIVERED" not the age, although it was mentioned.

Ben De Castro

It is not so much the age of the father but the age of the mother… just like today!

Paul Henderson