Sixt permits customers who are 21 years or older to hire a car, but there are certain vehicle restrictions and additional fees for drivers under 25 years old. If you meet the age requirement, you can book a car through the Sixt website or app, just like any other customer. However, please be aware that drivers under 25 may be subject to a Young Driver surcharge, which varies depending on the rental location and the type of vehicle selected. It is important to carefully review the terms and conditions specific to young drivers. If you have any questions or require further clarification, please contact Sixt directly.