No, I'm not proposing that at all, however, we certainly discourage people from having children at a young age. Military leadership could fairly easily adjust its recruitment practices, it relies far less on physical aptitude nowadays. Regardless, I don't really think this tangent is relevant to the discussion of college education.
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.
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.