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Here's a quick list of rules for renting a car:

• Must be 25 years or older - Lately, however, some car rental companies have been allowing 21-24 year olds to rent cars based on their previous driving records and with an underage fee.  Make sure to check the policies of your rental car company before making your reservation.
• Bring the car back with a full tank - Normally, car rental companies will charge extra for a returned rental that isn't filled up with gas.  So make sure to stop by the gas station before returning your rental.
• Valid identification - When renting a car, it is a necessity to have a valid driver's license.  Also, for some rental car companies, you must have a major credit card (MasterCard, American Express, etc.).
• Secondary drivers - Some car companies have no problem with allowing a secondary driver to use the rental.  But, the driver must be 25 years old.  If you get pulled over in a rental and you are underaged, then you're in big trouble.

All of these requirements depend on the rental car company itself, so ask your company's representative for information on rental agreements before making your reservation.

Here's some more information on rental car agreements and FAQs from different car companies: Hertz, Budget, Dollar, Thrifty, Alamo, Enterprise, Avis