One of the reasons that underage military members were no longer able to drink in the mess is because they were stupid enough to go to other bars and cause crap there. The change came around sometime in 1987. It wasn't too bad in Cornwallis at the Green and Gold because you couldn't go anywhere else. Then, when people got to Borden, they'd go to the Jr Ranks (or whatever the course bar was called), get drunk and then go to a civilian establishment. So, as usual, a few "bad apples" spoiled it for anyone else.
Should underage soldiers be allowed to drink? IMHO, no, it's illegal. Until the various drinking ages change, they'll just have to wait like everyone else.