Ironic that a 18 year old can join any of our military branches, be required to learn and use all kinds of weapon systems and serve in combat areas all over the world risking life, limb and possible lifelong mental anguish but can't legally purchase a firearm from a FFL until they are 21 years old.
Back prior to 1971 18 year olds couldn't vote either but were old enough to be cannon fodder and were only allowed to buy 3/2 beer on post.
Compulsory service at 18 started in 1942. Prior to that it was 21. In 1971 the argument was if you were old enough to be drafted you were old enough to vote, hence the 26th amendment.
I look at it like this. In generations past you were expected to act like a responsible man in your mid teens. You were still on "probation" under the control of your parents. Mommy and daddy held you accountable from a young age, but in your teens there was no tolerance for child like behavior. There was no tolerance to not controling your emotions.
This generation is encouraged to remain in a child like state for as long as they can. Young men in there late teens and early 20's still having temper tantrums. Oh, they may not be on the ground kicking, but it is a temper tantrum. They are taught not to control emotion, but to embrace and express it.
This doesn't apply to all, but sure enough to make the difference.
So I say no military service without parental permission, no buying firearms and no voting until 21.