It is not right to permit and/or encourage a young person to enlist in the armed forces at the age of 18 and possibly die for our country, then deny them the right to an alcoholic beverage.
At the age of 18, you have the right to vote and can be tried as an adult.
I've volunteered in an army hospital where service members under 21 were injured in combat and couldn't not be thanked with a simple beer.
If you're old enough to defend our country and die for our freedoms, you should be old enough to drink.