The age thing isn't an argument for maturity
I've seen just as many young kids cheat as young adults cheat as older guys cheat. I've seen young and old argue the most ridiculous things on the field. I've seen grown men whine like 12yr olds, and 12yr olds tell people to stop complaining.
You are who you are, and that's just how it is.
Most dangerous age seems to be 18-22 honestly.
The younger crowd is typically in better shape, but it's easier to teach the more milsim side of airsoft to older guys, especially the individual tactics.
I'd say any argument of age for maturity is invalid, it's all about how the person is taught and how they grew up.
So the main reason most people don't allow minors is for legal reasons.