I've seen some pretty big idiots in the military.
I actually laughed out loud when I read the title of this thread if that gives you any indication of my answer. In all honesty, it depends on the person, as it always does. My DB is very mature but there are a ton of guys in his company who do things like hook up with girls for fun unprotected and get them pregnant, make plans to marry a girl they met a week ago, buy cars with payments that will eat up their entire paycheck, and go out and get shitfaced drunk with no plan of how they're going to get back so they either call someone sober and responsible back in the barracks or drive anyways, get busted for a DUI, and get everyone called into work on their day off. (These are all things that have happened in DB's company in the past three months). One guy even decided that he was going to reenlist and where he was going to go by flipping a coin! THEN called his girlfriend to let her know what was going on and expected her to just go along with it! So no, guys in the military are not more mature. Not to say that they're less mature, but constantly being around young guys and alcohol will do that to you sometimes. You'll be better off judging this guy on his own and not based on his military status.
I would never date a young guy who just started out in the military unless I'd known him for ages beforehand and knew he had a level head on his shoulders. It seems like the younger guys in the military just get into a state of arrested development and bro out constantly/make horrible choices/draw penises on everything.