But they're mature enough to kill for and be killed for the national military (sometimes against their will), contribute to governmental policy decisions, and have children?
Just because their brains are still plastic doesn't preclude them from making good (or at least reasonable) decisions.
I don't think they are mature enough for that, no. We weren't arguing that part (at least not yet). 21 would be a much more reasonable age for that, but 25 would probably be best. There's a reason that you can't rent a car below 21 and that you have to make a significant extra payment below 25.
Are you also proposing preventing people from joining the military below 25? In terms of a male physical's peak, 25 is pretty late on in the game to start trying to train him to be a soldier.
You can shift mean college attendance age in a myriad of ways. Off the top of my head, bring public universities back under federal funding, reduce the for profit motives. Then start preferentially admitting students with a year or more of work experience as well as the current academic standards.
Most 18 year olds are nowhere near mature enough to take college education as seriously as it deserves to be taken.