Clinical fact that children do not have the maturity to responsibly handle guns until late teens. As "mature" as some may seem, their brains are still developing and "mature" they are not. By definition.
I knew a "very mature" 15 year old kid who, at a deer hunting camp, was cleaning his "cleared and unloaded" rifle at the end of the day. In a act of spontaneity that even he doesn't understand, he put the "cleared and unloaded" rifle to his heart and pulled the trigger. Just for "fun." The rifle had a round in the chamber. It was a miracle that he survived a point-blank 30-06 shot to the chest.
This is the problem with giving kids firearms. The act out spontaneously and without logic. It's what their developing brains do, and no amount of parenting can change that. Naturally, every parent will tell you otherwise. Denial is not just a river in Egypt.