Since the original purpose of the 2nd Amendment is defense of the state against a tyrant, any person old enough to enlist in the armed forces and bear arms, should, by definition be old enough to possess the same small arms as a civilian. I say small arms because heavy weapons like artillery and warships have traditionally been reserved to the standing services.
Sadly, it is in the nature of the totalitarian-progressive agenda to enhance the power of government over the individual whenever possible. And so the Left has put forth the proposition that all 18-20 year olds should be disqualified from firearms ownership. Even, and especially, the ones that didn't do anything wrong. They are simply guilty of their age. How many mass-casualty shootings have been committed by 18-20 year olds? Surprisingly, just 2. TWO. Reason enough to punish millions of innocent people.
A far better case can be made for changing the voting age to 21. Think of the votes cast by impressionable and misinformed youth! Think of the damage done to our country whenever the youth vote puts a bad candidate in office, or removes a good and experienced office holder. I'd rather see most 18-20 year olds armed to the teeth than standing in line at a polling place.
Seriously, if any legal adult is so unpredictable, so flaky, so unstable, so untrustworthy, so careless, or so deranged that he can't even be trusted with a gun, what is he doing running around loose? He should certainly be in custody.