Greylocks, you should be a bit more careful about calling something a law when it's really more of a community initiative. There are laws about the purchase and sale of airsoft, but not necessarily about ownership and where you can play.
Suffice it to say there are serious restrictions on owning and using your airsoft before you are 18, and you cannot legally purchase any guns before you're 18, period.
If your brother is willing to buy a gun for you, it may work out. However, there are SERIOUS consequences to the misuse of airsoft, and your brother would be well advised to understand what he's getting into by handing you an airsoft gun.