Why not write a basic program in Visual Basic (Likely won't take any longer than 10 minutes if you know what you're doing) that presents an Airsoft quiz to a person (Perhaps 50 questions?) that tests their knowledge of the related laws and the sport itself. At (successful) completion of the quiz (100%), it would give them a designated key to register an account on ASC (1 key could be used or a simple algorithm could generate them).
Summarized: Give newcomers a quiz and access to the information section.
While nothing is 100% effective, this would still help to weed out idiots (Except the ones willing to sit there and guess every answer).
Create a new petition system that asks a potential new user a few airsoft questions. A few designated ASC members can review the application and approve/disapprove the application. Approved applications would result in the user being given a key to register.
Banning ALL players under 18 solves 1 thing: Restricting minors from this site.
I'm sure there is many adults who are considerably irresponsible as well. Even if they're old enough to be considered legally responsible for their actions, it still doesn't do Airsoft (in general) any good when they fuck up.