And they are also considered legally adults, are they not?
I remember at 18 signing up for 5 years in the Navy. I'd say that is pretty adult. If it isn't, you should consider contacting your senator about raising the age of enlistment.
All the more reason to contact your Senator. Lower the drinking age or make the age of enlistment higher. Adult enough to decide to become a sacrificial pawn, but not adult enough to drink a beer.