if you try out one of the car hire "broker" sites, eg http://www.novacarhire.com/
if you enter your age and press "get quote", a pop-up will appear telling you'll pay more due to your age.
Hello isine, I think the majority of car hire companies have a surcharge for drivers under the age of 25 years.
I know that's true for Alomo, National, Enterprise - not sure about others.
Laura x