Most major car rental companies will rent to you but you will pay a surcharge of $25/day for being under 25. It will tell you when booking. Here is what it states on carrentsavers.com
Can under 25s rent cars?
The answer is yes. Major car rental agencies allow anyone over 21 to rent a vehicle. Most of the time there are additional fees, they average about $25 per day but in some states can be up to $75 per day. Certain contract ID's will waive this underage fee. Under 21s will have a more difficult time in renting but some of the smaller agencies will allow 18-21s to rent from them. It varies by location, if you have difficulties try our booking service and we will do the looking for you.
I think you'll have to poke around the various car rental web sites to see. I looked at Avis, and 21 year olds can rent normal passenger cars. My guess is they charge an additional fee which might be expensive. You might have to call to find out about fees. They will also run a check of your driving record and may refuse to rent if it comes up bad.
You might want to check with organizations you are affiliated with to see if they have a car rental program and if it helps you out as a young renter. For example, members of USAA (military) have the fees waived if the rental uses the USAA codes. If you have a parent who was in the military, you might be eligable for USAA membership.
If you are in college or are traveling with anyone that is, you could have them check to see if your/their school has any arrangement with rental agencies. For example my daughter goes to Pitt and she can use their codes to get the fee waved for extra charges on under 25 drivers. I even double checked with her school and you don't need to use it for company business. Just have to show ID when picking the car up. These rates are not always the best but may save you some fees. Hope that helps.