Most companies in the US have a heavy surcharge on drivers under the age of 25. What is worse is that it doesn't matter if you are 21 or 24.9 when you rent. What can be done by under 25 drivers to rent cars cheaply?
A good idea to escape the young renters' surcharge is to do your research well. The car rental business is very competitive and almost all agencies have discounts running all throughout the year, if one were to look closer. The most famous offer being the weekend discount. In fact, Hertz has a no young renters' surcharge offer running currently.
Another feasible option is to sign up on popular sites which list down rental discounts and offers from various industries.
When I was in Colorado years ago, we were investigating this, as most companies wouldn't rent to us (being under 25 at the time).
Back then, Hertz was suggested as a company that would rent to us, and potentially without a surcharge.
For example, they had this special available to waive this surcharge, so it won't cost you any extra to rent, being younger than 25. That one has now expired, but I also wouldn't be surprised if they and other companies offer these specials on a semi-regular basis - car hire is fairly competitive. Ringing them up and saying one of the other companies offered it might get you a matched offer from your company of choice - doesn't hurt to try!