Going to be highly dependent on context. At the cancer hospital? "Old Men" might just be 80+ years. At the office, it might be 60+.

Young adult in a lot of countries will start at 18 or even younger I think? US, adulthood starts at 18 even though a lot of adult things are still closed to them (drinking alcohol, having completed college, etc). So if we mean legally a young adult is probably 18-30 whereas if we mean a young person who is starting adult life we might not mean until 22 or older when they have a chance to start a career, etc.

Elderly and other descriptors might follow the contours of eligibility for government programs like social security.

I heard a teenager describing a 40 year old as "elderly". Context matters.

Judging by the telco tariff plans here I'd say young adult is 18-27

It's slightly variable and largely depends on the dragon's subspecies, although 5e removed all that in lieu of a simpler, but more boring, universal scale that covers all of them equally.

Adult's 25, Old's 50, Elderly is 75, Ancient is 100.

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.