> Does society deem control of adolescents an important social function because it expects that adolescents lack adequate impulse control given their destructive potential or does it not see such a gap and expect that they have adequate impulse control for self-containment?

There is nothing contradictory. Society holds children harmless because they're not generally cognitively capable of distinguishing right from wrong. Adolescents are, and are expected to behave. Society acknowledges that adolescents often have problems actually doing the right thing, but doesn't treat that as an excuse that mitigates culpability, as for children. Instead, they tailor punishment on a case-by-case basis to accommodate that fact. (Also, historically, they put adolescents to work, to reduce the avenues for misbehavior. Idle hands and whatnot.)