The self-imposed restriction of 18+ on airsoft guns is for at least a few reasons.
people below 18 are, in the large majority of cases, and i think by definition, immature. Airsoft is a sport that requires a lot of maturity and character, because there aren't always ref's watching you. You must act according to your better judgement, and moderate yourself according to accepted airsoft etiquette. I played CQB with some guys last night who displayed a complete lack of character by not calling their hits and ignoring the house rules(they were 18+ i'm almost sure). It takes integrity to have the will to compete and the desire to win, but to moderate your own behaviour in a game, and call yourself out when you know you're hit, but people wouldn't necessarily know if you kept going. a lot of minors don't develop any sort of responsibility or character till later on in their adolescence (according to my precise guesstimations). some develop it faster than others, and some just don't goddamwell develop it at all. The majority, though, are somewhat responsible by the time they reach 18. at least enough for our purposes.
second, airsoft is a physical sport that involves inflicting pain on your opponent. Nobody wants to shoot a 14 yr old. I'd feel bad (in most cases), and i think a lot of other people would too.
Third, airsoft mimics real life combat maneuvers. I don't know anyone who condones the use of children in a fighting force, and i sure-as-hell don't.
as a side, we want to put our best foot forward with the public eye looking at us (which it always fucking is), and having a bunch of children running around shooting replica firearms at each other is not only in bad taste, but it would have so many armchair moralists writing their politicians that it's probably make your head spin.