I believe they can hire a car at age 21, but agree they will have to pay a supplement up to the age of 25.

If they are going to drive, make sure they have fully comprehensive CDW on the vehicle. We ourselves always take fully comp insurance, even though we've driven in Florida for years. It is easy to get disoriented on the roads - day or night - and should the vehicle get any damage, you'd be fully covered. A major credit card would also need to be supplied by the driver.
Olga