An under 18 person CAN play airsoft and handle a gun IF;
The legal gun owner is present and monitoring it's use (and is preferably responsible legally for the minor), and the land owner allows it legally.
The problem we have is under 18 people wanting to buy airsoft guns and playing anywhere they please with no supervision or parental support.
Just like for real guns; you must be supervised and do it in a location that is appropriate. If you cant shoot a real gun in your backyard or your city, you cant use an airsoft gun either. For that bit, your age does not matter. It even applies to adults.
Yes, it's all very grey, but that does get applied.
Oh, right, what I have seen happen does not count. I'll let you all explain it to newbies so they understand.