I have been thinking about all of this stuff...I wonder how many of these people remember pre GCA68. I was one of those "infants" back then. I was over 21 by 68. My dad gave me my first .22 at age 12. He purchased it used at the local hardware store. I had to purchase and pay for my ammo myself with money earned from my paper route.
I presonally (no parent present) purchased my first Rem 700, and my first pistol, a Rugar Bearcat, at 16 with my own earned money at a actual gun store. My best friend had a .44 mag S&W, but then his dad gave him the money.
Guess what...up to 1968 "infants" were not a problem...responsible use was a product of the family...not government mandated training or restrictions.
Those that would use a weapon improperly, generally do not care what the law is. They do what they want to..law? they have no use for it, and still don't. The only people impacted by gun laws are those that will use them responsibily anyway.