I used to work at Enterprise rent-a-car and they do rent to anyone over 21. If you are under 25, there will be a fee. However, if you are nice and ask the fee to be waived, sometimes they'll do it.
Don't ever rent a car for your friend. In the rental agreement, your name is the one that will be held liable no matter what happens. If you don't put the additional driver name, and your friend involve in an accident, you will be held 100% responsible. Even when you buy the full coverage (insurance), that's only covering you and any authorized additional driver. If you let your friend drive it and he is not an authorized driver, the insurance is void.
If you put your friend as additional driver and he is under 25, you still have to pay for the fee. Basically, get your friend to rent it himself.