If your plans are uncertain, choose the “pay later” options when booking. You can cancel your rental for free if your plans change.
Some SIXT locations allow rental with a debit card and 2 forms of government-issued ID. It is best to ask about the payment policies at the branch you wish to rent from before booking.
You can rent a car from the age of 21 with SIXT in the US. An underage fee applies to drivers under 25, however. Some car categories have a minimum age of 25.