Well, you haven't seen the big picture yet. Many people leave school and an optional vehicle for work or a future is the military. There is not a thing wrong with that and may include putting ones self in harms way.
Bet you didn't know more deaths result from drinking under the age of 21, than the risk of a military casualty.
80% of alcohol related vehicle accidents are under the age of 21.
Alcohol itself has been fatal to people, even as young as 12 years old dying from liver failure.
To my know, on military installations, the age limit is 18 or older. Now if you want a jump start to drinking, go for it.
I just don't advise it. Keep in mind, off base you are subject to civilian authority, Leave base under the influence and driving is a double whammy, after civilian jail, you deal with MP.