I'm not sure i'm seeing the moral that you are trying to get across here Bojack. Is it that kids should go join the army if they want to be allowed to drink? Or is it that the only 18 year old who is responsible enough to drink is the one who joins the army?
Dealing with a large range of kids 5 days a week and seeing the ones that chose to go off to join the army at a young age, these aren't the kids that are responsible enough to make decisions like that. In fact i'd go a lot further than that and say that in general they tend to be the thugs, bullies and trouble makers that a) cause teachers no end of hassel and b) make their responsible and hard working peers (the sort i could entrust my wallet to at the start of the day and know i'll get in back intact at the end) lives a misery.
The kids that head off to join the army, in my experience, don't tend to do it out of some noble need to serve their country - the second world war idea of all our lads running down to the recruitment office to sign up to protect king and country simply doesn't hold up today - instead what we get is miscreants that think getting to shoot guns and kill people all over the world will be really cool.
As it is, the kids in your story aren't the responsible ones either, they are just whiny little piss ants that have latched on to some pop culture politics simply because it fits with their notion of how unfairly the world has treated them. I whole heartedly agree that they're not responsible enough to decide whether they should be allowed to drink or not and if they were allowed to drink would probably do something damaging or violent to the community. That however doesn't mean that those youngster that sign-up to the armed forces should be held up as the bar that the rest of our youth should strive to achieve.
Perhaps this is just a culture difference - maybe things are different in the states.
http://www.youtube.com/watch?v=UGJESEUYaQg
I've seen Doug Stanhope do a far better job of this bit and make a sharper point with it at other times, but this is the best i can find just now.
RJT.