The parents have become more irresponsible
Children who don't know pain, don't avoid actions that cause pain.
When I was a young child, i did stupid things, but I did them once.
I touched a hot stove ONCE.
I grabbed a hot iron ONCE.
I bit my mom, who responded by biting me back, ONCE.
I hit my dad, ONCE.
I put a cat in a toy boat in the goldfish pond, ONCE.
Why did I do these things ONCE? Because after I did it I was greeted by Pain. Either from my own doing or from the doing of my Parents.
When I was a young child, i did good things, and I did them alot.
I got good grades, A LOT.
I was good in sports, A LOT.
I behaved, A LOT.
Why did I do these things A LOT? Because after I did them, I was rewarded with pleasure. Either from my own self accomplishment, or by the praise and rewards from my parents.
At 27, I still feel that Pain and Pleasure are the only guides to my actions, but now, i can see beyond the next 5 minutes, which I could NOT as an under 18 year old. I can see that doing something stupid might not cause physical pain but would cause me pain in other ways. Because of this, I do not break the law, no matter how inane the law feels.
TL;DR
Guns should have no restrictions based on age once a person is determined to be an Adult. 18. I'd love to say that there should be no age restrictions at all, but parents are FAIL these days.