Allow me to play devil's advocate: Like abortion, there is absolutely nothing in the Constitution that prohibits young children, the mentally incompetent, the criminally insane, and persons convicted of violent felonies from possessing a firearm. Is it our position as the protectors and advocates the 2nd Amendment that government lacks the constitutional authority to prohibit anyone from owning a firearm? Or are we saying that government lacks the constitional authority to regulate the use and posssion of firearms by law abiding, adult citizens?
Hence the "reasonable man" standard.
This is not new ground.
If a parent wants to teach their kids how to shoot it's not the government's business. My stepfather, at the age of 12, walked into a Western Auto, put $12 on the counter and walked out with a single shot .22 rifle, and he never harmed anyone. The mentally incompetent, insane and criminals have traditionally had their rights restricted via due process through the courts. And some of them have had their rights restored by the courts. Congress doesn't need to make up any more gun restricted classes of people.