I think it’s more about the selection process and training than the age.
Those men are aways, AWAYS, under the direct supervision of an office are a mature supervisor of rank. You don't let a young private stroll around with an assault weapon and a pocket full of ammo they are supervised.
Uumm, you want to try that again? Maybe we can understand what you are trying to say. I think you are trying to say that they don't let the young solider go around unsupervised with weapons, which is quite true.
Although considering some of the officers, commissioned and non-commissioned alike, that I know, of several services (excluding the Marines, 'cause I don't know that many Marines and remembering that I *was* an officer myself) even their maturity is subject to question, as is that of engineers, programmers and pretty much all other so called adults, including myself at times.