For people under 21, their parents can always gift them a handgun. I got my first one when I was 12 or 13 and always had a pistol in the car when I got my drivers license at 16. I was also reasonable responsible and not a hot-head. I'm not saying I never did anything stupid, there was plenty of that, but never anything stupid with a gun.
But, to answer your question, I believe that if you're old enough to die/kill for your country, you're old enough for a gun, or smokes, or alcohol. I'd RATHER see them raise the limit for entering the military to 21 and leave everything else alone.