Normally there is no extra charge... but look here:-
The driver must have held a full driving licence for at least one year. At STEP2, click on the link "IMPORTANT INFORMATION' displayed to the right of the car photo of your choice. This will open another smaller window and if you click on "DRIVER" you will see what age restrictions there may be and any associated fees. Try a few different car models / suppliers, as they do differ.
For example Europcar say:- Drivers Age
At the commencement of rentals, main drivers must meet the requirements for licence eligibility:
For Mini, Compact, Economy, Intermediate and Estate car classes, drivers must have been eligible to hold a driving licence for 9 years.
For Full size, Standard or Passenger Vans or Premium car classes, drivers must have been eligible to hold a driving licence for 12 years.
Drivers who are 75 years old and older must contact the rental desk as further restricitions may be applicable.