I was under 21 until 11/2. I got into Zipcar when I was 20. The tip is walking up to one of their on the street tables, getting into the registration process and then saying "Oh wait, I'm under 21!" They really want to make a sale so they say "Don't worry, I will put you under the UC Berkeley plan". The only restriction is you can only rent cars around the school they added you to. All of the cars around them are nice cars but you won't be getting the Mercedes or BMW.
The on-campus limitation would also be a bit much for me. I'm living in Downtown Menlo Park which means that I'd have to go onto Stanford campus to rent a car. Think I'll just have to wait until I'm 21 :)
