You need to have a valid driver’s license plus an ID or passport to rent a car. Drivers with licenses that use non-Roman alphabets, like Japanese or Cyrillic, should also have an international driving permit.
In general, you can rent a car one way between SIXT locations in Madrid and elsewhere in Spain. This depends on the tariff, and prior approval is required.
The minimum age to rent with SIXT in Spain is 18. Some car categories have a higher minimum age for rental. An underage driver fee applies to renters under 23 years old.