I recognized that renting a car in the US is allowed at the age of 21 or 25. But are 18 to 21 year olds allowed to drive it if their parents or another 25 year old person signed the contract?
The age of majority in the United States is 18, and that is the theoretical legal age required to enter into a rental contract— but it takes two to agree to a contract. In practice, it may not be possible to find someone willing to offer a rental contract to someone under 21 depending on the
- rental agency
- rental location/market
- size and type of vehicle
- payment method
In the states of New York and Michigan, state law requires agencies to rent to licensed drivers 18 and over, though such drivers will be subject to heavy surcharges (sometimes prohibitive, over $100/day) and restrictions. Otherwise, none of the major agencies will rent to anyone under 21 except for U.S. military personnel or others renting on official government business.
Of the smaller agencies, Rent-A-Wreck allows 18-year-olds at some locations, Fox rents to 19-year-olds, and Ace offers that it may be possible depending on the location, and local agencies may also be willing to offer you a car.
"Additional drivers" are not exempt from minimum age requirements, and these will also vary by all the listed factors. For example, Hertz allows 20-year-olds, but National and Avis require 21, except in certain markets.
Zipcar is one possible (if very limited) option:
Some of Zipcar’s university partners offer the service to students, faculty, and staff aged 18 years and older. For participating universities, members must have a drivers license for at least two years and can have no violations on their records. (Source)