No-one believes that a law, in and of itself, prevents the behavior it outlaws. Laws draw a line around what we as a society think is acceptable. It is not okay (in most states, at least) for a ten year old to drive a car or drink alcohol. Doesn't mean it never happens: it just means that we've agreed we'll draw that line. Is it okay for a ten year old to use a gun without direct adult supervision? Should it be? How about a fifteen year old? We should have this discussion as a society.