There are reasons for the military to want older teens for training for war. But what you must remember is that when you are in uniform, you are required to respond as a member of the armed forces... including go to war and do unspeakable things. When you come home, leave the military, you remove the uniform and become a regular citizen again. That's one of the reasons for the uniform...to help with blending back into society when you can remove it. Of course the events and memories are another matter...but to help with it: "that was then" this is now.
Military training does in a way brain wash you to respond without having to think and decide at times. That isn't who you are, it isn't who you become, it's only what you might need to do when in battle.
I am reminded of a story about when it was common for hippies to call cops "pigs." Someone I know was called it once by a foul mouthed bigot, and he lectured the driver: that when he (the cop) went home, he'd take off the uniform, take a shower, put on clean clothes and sit down to eat with his daughter and wife, and he wouldn't be a pig anymore, however, the driver would probably be a bigot all his life!
We are not what we go through. Our experiences do help shape who we want to be...it's up to us to constantly battle the negative and emerge victorious members of society.