It's extremely slippery slope this topic. RKBA is a right. At what age do rights start? Does the Constitution not apply until the brain is fully developed? Who's going to determine when my brain is fully developed and when I can "good" decisions?
If you ask an anti, they'll say that NO gun owner's brain is fully developed. Who sets this standard? Rights were not meant to be able to be taken away simply because someone else deemed that we didn't "need" something, or didn't "need" to do it.
I've been in the military for almost 20 years. In that time I've seen a lot of 18-20 year olds. Nushif is right, military guys tend to play a little harder than their average civilian counterpart. Does it matter? I don't know. I can tell you, the difference between this 18-20 year old and that 18-20 year old can be immeasurable. No 2 are alike and they mature at different rates. It's inconceivable to be think that we can implement a "one size fits all" solution and it not take away the rights of someone for whom it was perfectly safe.
Many of the people that I know in that age bracket, even the ones that grew up around guns, really don't want to concealed carry anyway. They've got other things on their minds.
Those that would give it more thought see it as more of a danger than the prevention thereof. This is of course from a military perspective and CCing is considerably more difficult for these young adults anyway, it should be of no surprise that the numbers would be low. As my own children have grown up and I see the same age bracket in the civilian world, I really haven't seen THAT big of a difference. They're typically ten foot tall and bullet proof with other things on their minds. I think that those that do want to CC are in a considerably smaller minority than the adults with "fully developed brains."
That small group of young adults that are CCing, or would, have at least acknowledged that there is a law, they've at least attempted to follow that law thus far, they're willing to pay for their permit, and at least become somewhat educated on the matter. I have no problems with them carrying a gun, those aren't the guns that I am worried about. Just remember, it's tough to get back a right that you willingly allows to take away from someone else. We should all tread carefully here.