My dad was old military and all of my guns were securely locked up and he had the only key to the rack. If I wanted to use one of them, I asked and he guided me very closely until I was 14. He still watched me after that but he trusted me and knew I had learned the lessons to responsibily use firearms. I raised my kids the same way and they, both in their twenties, are very careful with guns. They also know I still watch them like a hawk when they are shooting. I guess old habits are hard to break. I'll be watching them when they are in their 50's too.
As a school teacher, I see a lot of kids I wouldn't trust with a plastic butter knife, much less a gun. We have age limits on buying firearms and to my mind, they are pretty rational and fair. At the same time, I know older folks I wouldn't trust with a gun either. It comes down to the individual. Gun ownership comes with responsibilities and if you can't handle it, no matter your age, you don't need them.
I am very much pro-gun, but I also realize there are idiots in this world too.