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.

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.

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.

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

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.

lmfao ancient? “Ya, I’m pretty ancient myself”

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