According to the Federal Operating Regulations (BGBI. 951) you must meet the following requirements in order to be allowed to drive a taxi: 

Group B driving license (65, para. 1 KFG 1967) (no provisional license)

• Car driving experience: At least one year before the issue of the taxi driver identity card
• Minimum age: 20
• Certificate of "First Aid Course" (at least 6 hours)
• The application will only be accepted if there is no entry in the criminal and traffic police record (grossly negligent and repeat offences) in the last 5 years
• To be admitted to the taxi driver exam, there must be proof of the completed training