Visiting the United States, under 21, and looking to rent a car or other vehible? Usually no problem, as the age for rental is generally 18. However, keep in mind that car-rental companies try to protect their cars in the case of relatively inexperienced drivers by imposing young-driver fees and making only economy class cars available to them.
Furthermore, car rental under 21 with no fee is possible if you know where to go and what to do. For example, if you're traveling with an older friend or family member you can ask him or her to rent for you. You can also buy a membership from the rental company to have the young-driver fee waived.