How come accountability with minors only happens by way of violence?

It seems rather disingenuous to me that a young man such as Brandon McInerney would be seen as culpable and responsible enough to be tried as an adult because he shoots and kills someone under the age of 18, but not if he has sex under 18 with anyone consensually; especially if his partner can be held accountable when he equally consented (be it seen as legal or not). If he ended up in the bed of his over 18 female math teacher only she would be the one seen as culpable. If he can be seen as too ignorant or stupid to agree to be there with her because he is under 18yr. how does he become more intelligent or able just because he uses a gun that somehow ends with a death? Why it is that violence will get a minor seen as culpable when they can actually be just as able, and capable in deciding with whom they have sex? Some will try to cleave the fact one act harmed no one, (or him in some way) so he can’t be culpable, or that there was harm to another but, the principal is still untouched; that is still not enough change if one truly understands the gravity of their actions.