Consent is agreement to engage in certain activities given after consideration that requires developed reasoning and ability to make informed choices. Obviously, a young child lacks both the ability and the informaton, when it comes to sex, gun ownerhip, driving, military service, voting, etc. An 18 year old is supposed to have them. Of course many still don't. But there needs to be some, however imperfect and artificial, limit. The only alternative would be some system of complicated tests. Just imagine how many people would never get to vote or have sex.