It's not about age, but about how much you want to spend. I suggest you come out to a low key event and see what people use, and ask to try it on. That way you'll know what you want before you buy it. Or you can go to Airsoft Outlet NW in Cornelius and try on some gear. If you're in Washington, there is ThunderKiss NW (airsoft store) in Vancouver, to try stuff on.
There are some indoor areas, and there are a few fields. Local field that is regularly open is
http://www.camprandr.com or two indoor arenas (one in Hillsboro, one in NE Portland).
My advice is to go to a game, try stuff on and weapons out, then ask in person about what you want.