In general you're going to find that car rental agencies will include a surcharge for drivers under 25. In many cases it's actually a matter of law of the minimum age for renting, so it's a matter of company policy and the jurisdiction you're in.
After some quick googling, I found the underage driver surcharge list by state for Budget car rentals. If you scroll down to Florida, you'll see that for most jurisdictions, there's a surcharge for drivers who are 21-24, which implies that is the minimum rental age in that jurisdiction. However some cities like Orlando indicate a flat "under 25" surcharge, which implies that the minimum age may be 18. If I were in your position, I would find a likely candidate company and city -- such as Budget in Orlando -- and call them.
E-Z Rent a car will rent a car to 18-20 year olds with a $35/day surcharge but there may be additional policies in place based on jurisdiction. Ref
Avis has a minimum age of 21.
Fox will allow you to rent if aged 19 or older. Ref
These are just the first few results I found when searching. I'm sure you can find others; see the note below on search terms to use (American English differs slightly from British English in this regard.)
For reference in doing further research, we Americans refer to it as a "car rental" (the concept), "renting a car" (the action), and the "rental car" (the result. We tend to use the verb "hire" to refer to persons being employed in tasks rather than objects being 'hired' for a time.