I used to think that but for "anyone born in a year after my birth year", but now I look at some such people and realize that all people eventually turn older. :/ OP is a lot younger than even that though.
Yeah, come on, of course. "Young" = no one would question the chromosomal health of your baby if you chose to have one right now. So it's in the 30s when that starts to get shaky, but certainly not the 20s.
I dunno, to be honest. But for me, reproductive changes are a consequence of age, not the other way around. No single factor feels like it in itself is a sensible definition.
But when people say, "You're not young any more," it's usually a reference to the biological clock. At least, to me the science strongly suggests that the cutoff is age 35: https://en.wikipedia.org/wiki/Age_and_female_fertility
Not my experience, but that might depend on the people one interacts with. I'd also say that coupling being young to such a gendered parameter is questionable. When does someone who's biologically male stop being young? What about people who can't reproduce?
A person at age 40 (or 30 or any age) isn't suddenly considered to be younger by anyone just because they're infertile, in my experience. But heck, I don't know; with all the anti-aging research going on, maybe we will be successful at ending age-related mortality in the next couple of centuries, and "old" would literally become just a truly relative number. For the time being, all I can say is that in my family and circles, about 35+ is considered not-young for any human, and especially 40+.