This depends a lot on the context and who is judging.
For example, a 50 year old would be deemed young by someone who is 90. I think you could call someone young who perhaps has spent roughly half the amount of time on this world as you have.
So teenagers are young to mid twenties and 30 year olds (as well as everyone up)
If you're looking for a general term for young, I think I would personally go for up to 30 being young. But again, your as youthful as you feel!
I know my uncle is 47 and yet he seems to have more fun and has more youthful views than some of the 21 year olds studying the same degree as me!