The age thing annoys the crap out of me, simply because I wish they'd pick just one damn age for adulthood and be done with it. I don't give a rats ass if it's 18 or 21, but pick one! Vote, smoke, own guns, join the military, play the lotto and drink. Pick one. The end.
Although, how the hell they justify 21 for all firearms purchases but will put an 18 year old through Basic and ship them off to a war zone to kill people with a gun is just beyond me.