if we have to give a definitive answer, then i agree with you. the problem, is that 18 years old is an arbitrary number. some kids mature faster than others, and vice-versa. some, "kids" have the maturity of adults and are aware of their actions, and the damage that can be caused by those actions. this is why once a kid is, say 14 and above, i think most would say that they are self-aware of their actions, but are still legally considered minors. kind of messed-up to treat someone as a minor when the minor in-question is clearly thinking like an adult.
to me, it all has to be on a case-by-case basis. that's best. leave these things up to juries to decide. that's why they're there. to deliver the, "general will" of the people.