I haven't been 21 (or 18) in a couple decades, but I remember impulses.
Of course, my impulse control still may be lacking a bit, at least when it comes to online ammo purchases or drooling at the LGS...
The thing is, though, I have a problem with age of majority and status offense laws. My take on it is, if the person would be charged as an adult for the crime, then the person should be treated as an adult, period.
In other words, if we want to have drinking age, carry permit age, etc predicated on age 21, then maybe we should move the vote back to age 21, too, and treat 20 and younger as juveniles across the board.
Given that the argument still stands about 18 year olds being draftable, and serving in the military, I don't think that would work. (Nor, frankly, would I advocate it anyway.)
So, my take on things is, if they incur adult liabilities at age 18 (criminal charging; ability to enter into binding contracts; etc) then they should receive full adult benefits at age 18.
Otherwise, older folks beware - because some of the stats about people under 21 being more dangerous in certain areas could come around in other categories to bite people over 65, or 70, etc.
"Hey, it's for the greater good - a 75 year old man should not be allowed to drive a Corvette!"
(Assuming, of course, the 75 year old man's hips and knees will let him enter, exit, and sit in the 'Vette in the first place...)