It is possible to rent a car one-way between SIXT stations on Mallorca, but not between stations on the island and Spain’s mainland.
You will need a valid driver’s license and an ID or passport for a SIXT rental in Spain. Those with licenses using a non-Roman alphabet like Cyrillic or Arabic will also need an international driving permit.
The minimum age for a SIXT rental in Spain is 18, but some car categories have a higher age requirement. An underage driver fee applies to those under 23.