Just like with the gestational limits on abortion, there cannot be a perfect answer. We all understand that some people become mature and responsible at 13, and some - well, never. Whatever is the current cultural consensus if fine with me.
But the current American laws regarding the matter are not fine at all. How come a person is trusted to make super-serious decisions like joining the military (risk killing and being killed!), and to vote, and to marry, etc by 18, but cannot smoke or drink until 21? Having a glass of wine requires more maturity than combat duty?