Well, we could turn this on them (Dems/Left). You should be 21 to vote. You know because folks that aren't 21 or older, are immature and prone to making bad choices.
It is more that people are more impulsive until they hit their early to mid 20s. That said, I'd rather see the age be 18 with no age on possession. Your parents think you are responsible enough to possess a gun at 7, that's on them.
I also think your parents should be legally responsible for you to, so if they made a bad choice, they share any punishment/blame for the consequences. Since the consequences are exceedingly rare, seems like most parents make the right choice.