The meaning of Young Dumb and Stupid refers to teenagers or those under 18 years of age that do stupid and dumb things because they think it is cool and helps them fit in.
However even some older people can do stupid things so it's not just young teenagers that do young dumb and stupid things.
Actually not every teenager is stupid or does stupid or dumb things and not every teenager is a thug or bully but some are.
When I was a teenager I had teens in my neighborhood that would make fun of me, call me names try to pick a fight etc.
I never stooped to their levels though and just brushed them off.
I was never that way so not all teens are like that but a lot of them are.
Eventually they will grow out of it, hopefully find a job and be away from you anyway and they'll eventually forget about what they've done.