I'm 17 and I'm perfectly capable of understanding that stealing, assault and killing is wrong -- underdeveloped brain and all.
I don't see any reason to harshly punish kids for silly stuff like smoking pot and graffiti. The law says it's wrong, yes, but teenagers are going to do it anyway.
Things like murder (intentional, premeditated murder) are not excusable in an older teen because "they didn't understand it". In a 10 year old, maybe, but a 16 year old? Nuh-uh . . .