The monthly car rental options for an 18 year old will be hard to come by...and
expensive! It's difficult enough to find good monthly car rental deals (See our Tips to Save Money on Long Term Rentals), without the younger age being a factor.
The majority of car rental companies will not allow someone to rent at 18 years of age.
The usual minimum age to rent a car is between 21-25, with underage fees (avg $20-$30 extra per day) for those under 25. The minimum age for an additional driver is 25.
Rent a wreck is one company that often rents to underage renters. Their website policy on age states "Usually, Rent-A-Wreck requires renters to be at least 18 years old, but some locations may require a minimum age of 25 years. For consumers under 25, a credit card is usually required for payment and the rental rate may be higher." Your best bet is to check some prices for some good monthly deals, but I have a feeling that the underage fee will inflate the price of the rental significantly.
You are required to contact the individual location for information on their minimum age policy and underage fees. You can visit Rent A Wreck.com to access those addresses and phone numbers. Most location listing also include an email address.