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?
I would say “young” is anyone 3/4 your age, or less. “Old” is 5/4 - 6/4 your age or higher. Though the fractions can vary from person to person.
Elderly is generally retirement age, or close to it. The point where the body’s slowdown becomes obvious.
Ancient is roughly 80+ those who have exceeded the average human lifespan, and are still ticking. Though elderly + frail is how it often plays out in public, since you don’t know someone’s actual age.