It's too bad there can't be some test for maturity other than chronological age that the law could use, but unfortunately any such test would probably be used by the criminal gangs in government to disarm as many civilians as possible. So age about has to be used, as flawed as it is. Maybe some things could be added though, like you have to be a certain age and have a high school diploma and no record as a delinquent, for example.
To me driving is at least as dangerous as being armed. In fact it can be more so; there is no danger in texting while carrying a gun, for example. So the age of driving and the age of exercising our right to be armed should be the same. I would place that age at 18, and I would add having a diploma and no record; otherwise the age for both would be higher.
I remember when I was in junior high school in the mid 60s it was not uncommon for boys my age to have our own .22 rifles and .410 shotguns, and go out shooting cans or rabbits, with no adult supervision. And before 1968 we could buy a gun mail order. But we still had less crime then than we do now, including so-called gun crime.
Clearly the problem is not guns, it's rampant liberalism. But that's a subject for a different thread.