Moreso if you believe they're intentionally seeking recruits at a younger age to ease said brainwashing.
The thing is, this is exactly what the military does. You may not like the term brainwashing, but in reality that's what they are doing. The average person joining the military is not a killer, but that is specifically what they need to turn you into. You need to be able to put a child in your sites and pull the trigger if that's what is needed. Pick some random person off the street and tell them to do it, more often then not, they would die when the kid blew them both up. It would be rare to find someone who would actually pull the trigger. The military needs to train you to do just that, pull the trigger and take the life. During bayonet training you may hear the exact words "Your a killer", they will want you to act like one. If you do not think the purpose of BCT is to start to mold average citizens into killers, you are deluding yourself. The whole purpose of it is to acclimate you to the military life. The ultimate goal of the successful military must always be to achieve peace, however, to do this you need to be willing and able to take a life.
In short, they seek out younger people for this because they are easier to mold (brainwash) into what they need, people willing to kill. There are certainly other reasons the military targets teenagers, but the primary reason is that they are easier to turn into what they need.
For to win one hundred victories in one hundred battles is not the acme of skill. To subdue the enemy without fighting is the acme of skill.