It's to keep liability down. The insurance pool that ZipCar benefits from would be significantly skewed towards youth and impacting the majority of their client base to generate more credit-risk revenue is not part of the model.

An 18 year old is at least twice as expensive as a 40 year old business man when it comes to car insurance.

The age limit doesn't apply if your Uni has a contract with Zipcar — many of my under 21 friends are Zipcar members . . . and its great!

Why? Most rental companies charge extra fees, or don't let you drive, if you're under 25.

Yes, but most college students aren't doing independent long-distance travel. The 40-year-old businessman can probably afford a car and almost certainly needs it frequently; the 19-year-old college student has very infrequent and short-term (but inescapable) requirements, for which Zipcar is perfectly suited.