Because they are getting trained.
There are plenty of "country folk" who are just as trained and perhaps even more so (prior to turning 18 b.t.w.), than many "city slickers" who get 10 - 16 weeks of training in basic.
18 is the legal age when one becomes an adult. Of course one size doesn't fit all, but current politicians need to think twice about taking away everyone's constitutional rights based only on the fact that they are 18 or 19 or 20.