Nevermind the legal bull just plain common sense :
If it looks like a real one, treat it as such.
There have been problematic situations where a policemen drew his service weapon on minors going in the streets with these so imagine an instant that same policeman shot those kids, that could be your kid and that policeman could be an uncle stuck with that nightmare for the rest of his life.
There has been a story or two about 3 boys assaulting a girl in a park with those. She could have had an eye shot out because these things but luckly not. You need a safe and propper environnement for those like a paintball field or at the very least, a private field where there is no trespassing.
It's a good idea to look around and get info, most people dont and it just ends up crapping on the people like us that actually go the extra mile, literrally, and do the thing in a propper and safe manner.
So yes, minors could play with these safely but they require supervision at all time with facial protection and a propper controled area like paintball only.
Having a maximum limit of FPS for them is a good idea too in case the thing ends up out of you control for X reason. I suggest looking up airsoft related pictures of bb incidents.
In all honesty, I think these are toys for big boys only and should not be used by minors at all.
Certified Level 2 BA Sniper
Si ton épée est trop courte, allonge la d'un pas. ( Proverbe Hongrois )