Let’s do some math. Sarah is 10 years younger than Abraham (Gen. 17:17). To keep things simple, let’s just assume it’s 10 years to the day. Abram was called by God at the age of 75 (Gen. 12:10), meaning Sarai was a youthful 65. Some time afterward, but less than 11 years (Gen. 16:16), Abram goes to Egypt. Sarai is somewhere between 65 and 76 at the start of this trip. Abram becomes Abraham at age 99 and Sarai becomes Sarah at 89. Abraham moves to Negev sometime during his 99th (Gen. 20:1) and Sarah’s 89th year. Isaac is born when Abraham is the remarkable age of 100 and Sarah 90.
I bring this up because on two occasions Sarai/Sarah is more or less given away as a bribe to powerful men. Pharaoh (at least his citizens and officials) find her beautiful (Gen. 12:14) at 65 (possibly as old as 76). Considering the unbelievable lifespans of everyone that lived in Genesis this could work. Sarah lives to be 127, which when normalized to a 90 year life span puts her somewhere between 46 and 54. I’m making two assumptions here: that in the times of Genesis the rate of aging wasn’t delayed, but rather slowed down, and that Sarah didn’t die prematurely. This is still a believable age for captivating beauty.
What is harder to believe is the second (documented) instance with Abimelek. When Sarah is taken by the King, she is 89 or 90. This would put her at about 63 if normalized to 90. I’m not sure what interest the king would have in taking an old woman like Sarah. Novelty? Fetish? Perhaps the senior discount?
Maybe my assumptions are off base and my relative age estimates are wrong. Regardless, the author of Genesis thought Sarah to be “very old” and “past the age of childbearing” (Gen. 18:11) at the age of 89. It’s doubtful Abimelek would disagree. Maybe that’s why he hadn’t sealed the deal with her before God threatened him with death (Gen. 20:3-7).