OLD ENOUGH TO VOTE (and get killed in combat) but not old enough to smoke.

I think there ought to be a single staged process for formal entry into adulthood. Say 16 to drive; 18 to drink and/or smoke and 21 (or high school graduation, whichever comes first for full legal rights (entry into contracts, military service, etc.), except perhaps voting where I could be persuaded to require some kind civics knowledge test before being allowed to vote..