Obviously teenager is 13-19.
“Young adult” would start at 20, but where’s the cutoff at the upper end? Similarly, what’s the range for “adult”, “old”, “elderly”, " ancient"?
If someone asks for responses from “old men”, how do I know if it applies to me?
There’s going to be some overlap and it’s very subjective depending on who you ask and the context, but in general I’d personally say
Infant: <1 year old
Toddler: 1-3
Child/kid: either used generically to refer to any minor, someone’s offspring, or about 4-11ish
Tween: 10/11-12
Teen: 13-19
Young adult: usually 18-about 25, depending on context may include older teens about 16+ and go all the way up to 29
Adults: 25/30 depending on where you end the young adult range- 39
Middle aged: 40-60/65ish
Old is very subjective, and depending on context, the person’s personality and health, etc. I wouldn’t normally use it to generically refer to anyone under about 50/55, and more often probably skewing more towards 60, 65, even 70, though I’ll occasionally use it half-jokingly for people about 10 years older than I am, which would currently make those “old” people in their early 40s.
Senior- 65+
Elderly- 70+
I would normally only refer to a living person as “ancient” jokingly, I’d normally only use it seriously to describe things that are from probably about the 5th century CE (or AD if you prefer) or earlier.