All drivers need to be at least 21 with a driver’s license held for at least 2 years. Some car categories require renters and drivers to be 25.
You will need a valid driver’s license held for at least 2 years, plus an ID or passport. If your license comes from certain countries or uses a non-Roman alphabet like Arabic, you will also need an international driving permit.
One-way rentals between SIXT stations are only available on request. Contact the branch you want to rent from.