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.