We are told in Genesis 21:5 that Abraham was 100 when Isaac was born. In Genesis 17:17 Abraham doubts that a child could be born to someone of 100 years old. But we are also told that Adam was 130 when he fathered Seth, Seth was 105 when he fathered Enosh, Methuselah was 175 when he fathered Lamech, Lamech was 182 when he fathered Noah, and Noah was 500 when he fathered his three sons. Abraham's own father was 70 when Abraham was born, which would still be considered old be modern standards. So it appears that having a child in one's old age was not an uncommon thing. Why, then, would Abraham and Sarah doubt their ability to birth a son at 100?
Earlier, God pronounced a reduction on human longevity in
Genesis:6:1 When human beings began to increase in number on the earth and daughters were born to them, 2the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose. 3Then the Lord said, “My Spirit will not contend with humans forever, for they are mortal; their days will be a hundred and twenty years.”
By the time of Abraham, it was extremely unusual to father a child at 100 years old.
Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?”
The word "doubt" is not found here. He laughed.
The answer here is as simple as the recognition of the biological fact that Sarah was menopausal and past the age of child-bearing as recorded in Gen 18:11
Abraham and Sarah were already very old, and Sarah was past the age of childbearing.
The ancients understood that when women reached menopause, no conception was possible under normal conditions. If Sarah was to have children, it would be a miracle - it was such a possibility of a miracle to which both Sarah (Gen 18:12, 13) and Abraham (gen 17:17) laughed. They we both rebuked by the LORD for this lack of faith.
I often think that perhaps God deliberately waited until Sarah was wellr past the age of child-bearing to make the miracle very obvious, and thus to ultimately strengthen their faith.