It depends if you're talking in legal terms or not. Actually, it doesn't really depend on that at all. But I don't think that the government should dictate what people can or can not put in to their own bodies. I personally think that, in an ideal society, there wouldn't be age restrictions for anything and that people take responsibility for their own actions.

In the case of minors, however, I think it should be up to their parent/guardian (i'm speaking hypothetically, of cours and assuming that, in this scenario, Cannabis is legal) to decide.