Trust me, those 18 year old soldiers are old enough to fire a weapon and go to war, but a lot of them are far from mature adults. From the first time they step off that cattle car at basic to the day they get to their first unit, discipline and doing the right thing is beat into their heads.
Once they get to their first unit, it's a whole different world from what they got in basic and AIT. They don't have that 24/7 supervision that they had before and a lot of them now have more money than they've probably ever had before and once they are away from work and the squad leader or platoon sgt is telling them what to do all the time, it's now time to see what he can get into. Now it's time to get his new buddy that's 21 to run down to the Class VI (liquor store) and see if he can buy out the store. Or better yet head to one of the local bars and see if they can drink it dry (most of us do love to drink).
I'm not knocking the Army or the military (well unless you were a part of the United States Social club, i.e. USAF) since I spent almost half my life in it, but we were far from angels either. An 18 year old soldier may be more disciplined when it counts and he may have to lay his life on the line, but they do love a good party and if their first duty station is Korea, they will learn more about partying in that year than any college student will learn in four years in school. Oh and I was only kidding boys in blue, we only kid because your idea of roughing it is having to stay in a 4* hotel instead of a 5* hotel when deployed....