[Carries around a sword, gets into sword fights, kills wildlife/engages in poaching, theft of personal property, attempted homicide]
"I'll give you booze."
"What?! I'm underaged! No way, that'd be ILLEGAL!"
I got news for you, bub. Even if you were still in Japan, underaged drinking would be the least of your concerns.
Although, sadly, a lot of this is censorship at work. If you have anything that is encouraging the "moral degradation" of the youth or contributing to the delinquency of a minor, your editor is going to make you remove it. Especially in things like Jump and other shounen focused magazines.
Oh, but if you want to make your protagonist an edgier-than-bismuth psychopath that enjoys murdering people to sate his ego, that's fine. Just don't you DARE make him drink a sip of alcohol under he's 20 (legal drinking age in Japan) or have him with a passenger on his bicycle. That's too god damn far, you hear.