Car rental companies do not charge a higher rate for opitional insurance purchases,
like the Collision Damage Waiver
The rental car company will charge an underage fee for any renter
fee is per day on top of the base daily rate of the car rental itself.
Read more about Underage Fees
I went to discountcar.com and checked some rates. The terms and conditions (underneath the rate quote) stated that all rates on the site are quoted for only for licensed drivers who are aged 25 and over and that hold a full, valid driver's license.
So, there is no insurance rate for car rental when the renter is under 25; this company does not allow any renters under 25.
For Discount Car renters 25 and over, the optional CDW (Collision Damage Waiver) varies in price from city to city. You would have to check a rate quote for the Discount Car Rental location that you want to rent from to determine the price of any optional insurance purchases.