I had a hottie in the rotation that was for recreational use only but hyper-attractive and intelligent enough to have conversations with that were entertaining. She was in her early 40s and had never been married or had kids—held a promising career, etc.
Near the end of our affair, she began talking about not having kids and how it was gnawing at her from the inside. She was not delusional, thinking she could still have them, but instead, she hadn't, and that was her greatest regret. She was a selfish and hedonistic smoke show. She'd been around the block more times than I'd like to admit.
I've been involved with several women in this age range who are childless, and all of them - to a woman - regret not having kids. They were all 7+ and, as a result, got to party it up in their younger years, not realizing that the party would end eventually, and it did.