In the book of Genesis, after God tells Abraham that he will have a son through Sarah, he makes the following remark:

Genesis 17:17

17 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?

This makes little sense for the following reasons:

• Abraham's own father had him when he was 130 years old (Genesis 11:32, 12:4, Acts 7:4)
• Abraham's father died at 205 years old (Genesis 11:32), and at the time of the passage in question, Abraham knows that. So it would be reasonable for someone in Abraham's situation to assume himself to be middle-aged
• Abraham himself lives to be 175 (Genesis 25:7-8). His son Isaac lives to be 180 (Genesis 35:28-29). His grandson Jacob lives to be 147 (Genesis 47:28), and had his son Joseph at around age 90, and Benjamin even later. This would place Abraham's age of 100, again, near middle age, and would make him siring a child at 100 totally unremarkable. Abraham apparently sires at least 6 more children after Isaac about 35 years later, which barely receives a footnote (Genesis 25:1-2)
• At the time of Abraham, there would have been multiple generations of 300 and 400 year-old people walking around and in recent memory, according to the genealogies in Genesis 11:10-26. According to those genealogies, Noah himself lived to be 950 years old, and would have died just a few years before Abraham was born

Therefore, Abraham's comment in Genesis 17:17 makes little sense. "Shall a child be born to a man who is one hundred years old?". Yes, frequently. Virtually every person who Abraham would have been aware of in his genealogy, and presumably in his locality, would have considered that to be nothing special, including his own father.

Likewise, Sarah's laughing at herself being able to conceive, specifically citing the fact that "my husband is old" (Genesis 18:12), makes little sense. Her husband was about middle-aged. He had only lived about 50% of his father's lifespan, and as it turned out, about 57% of his own lifespan. Similarly, Paul and the author of Hebrews' comments about Abraham's great age (Romans 4:19, Hebrews 11:12), in light of these things, make little sense.

This oddity lends to the idea that the "Abraham's great age of 100" narrative was originally separate, and inserted into a different narrative which contained age-related statements about some of the same figures. But, when they are combined, the "great age of 100" narrative actually does not make sense in the context of the rest of the book.

Also, consider the legendary nature of Abraham being exactly 100 years old when Isaac was born (Genesis 21:5). 100 is a nice round number in the base-10 counting system. In picking exactly 100, the story teller gives it the air of a tall tale - "The legendary figure was old. How old? He was, 100 years old!". This is akin to having a giant in a story - "He was tall. How tall was he? He was 10 feet tall!" - not 9 foot 9, not 10 foot 3 - 10 feet tall. The number 100 is memorable, and clean, and perfect as a symbol for someone in a story who is "very old".