No, you must be at least 25 years old in order to rent a vehicle at First Rent-A-Car
No temporary licenses are required for those holding a valid U.S. license. Temporary licenses are available for those holding a non-U.S. licenses for a small fee. Out of territory licenses are valid for 90 days after arrival.
We are creatures of habit, so yes if you are used to driving on the right side of the road breaking the almost instinctive urge to be on the right side takes some adjustment. Most visitors report that after a few minutes they get adjusted. It isn’t difficult to drive on the left, you need only remind yourself to stay on the left – particularly when turning corners and exiting a parking lots
4wd is not required on St. Thomas. On St. John it is often recommended, however 80% (rough estimate) of the island’s roads can be navigated with out it. Some villas on St. John are accessed by steep, sometimes dirt roads which make 4wd a must.
Parking is easily found. There are a few exceptions. On St. John a few beaches have very limited parking (Maho Bay, Jumbie Bay, Gibney are examples). Also Cruz Bay, the main ‘town’ on St. John has limited parking.