Do any of the rental car providers allow 21 year olds to drive the vehicle?
no, not many but check 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's age" you will see what age restrictions there may be and any associated fees. Try a few different car models / suppliers, as they do differ.