Most car rental companies these days will gladly rent to anyone over 21--but note that if you're under 25, there's usually an underage driver surcharge to deal with.
(Some states have laws requiring rental companies to rent to even younger customers--New York, for example, lets anyone who's at least 19 rent a car--but the fees for that are sky-high.)
It's sometimes possible to reduce or get around some of these fees. While we don't yet have an automatic way of handling rentals with underage fees, you might want to check out the "wikipost" at the top of this discussion thread on a popular frequent-traveler message board for some helpful tips and tricks.
If you find anything there that you might qualify for, then the best thing to do is to click here to request a price quote, and then when you receive the quote email, reply to the quote and let us know what memberships mentioned in the wiki thread above that you might have. That way, we can help you get the very best price possible for your rental car!