I'm sorry to tell you that you are going to have to wait until you are at least 25 to do the trip you are dreaming of.

Buck mentioned the underage driver fees, and those are something you will have to worry about before you can take any roadtrip. However, even there, until you are at least 25, your choice of cars are going to be limited to base model sedans.

All rentals in the Specialty Collections, like the Mercedes you mentioned, have a minimum age of 25, with no exceptions. Also, specialty collection rentals are typically not available for one way rentals, so you wouldn't be able to rent one in Florida and drop it off on the other side of the country.

Clearly, you've got a lot of time before this trip will happen, but that's a lot of time to keep dreaming and thinking about what you want to do when you'll finally be able to reach this goal.