According to the vaccination ordinance of the Federal Ministry of Health, everyone in Germany is entitled to a booster vaccination. The distance to the previous vaccination is not specified in it - not even the age. The federal government does not make any binding specifications in this regard.
The decisive factor for the question of who is liable in the event of vaccine damage is the approval of a vaccine. For the booster vaccinations, the EU Commission has so far approved the mRNA vaccines from Biontech and Moderna and the vector vaccine from Johnson & Johnson approved. For Biontech and Moderna there is a booster approval from the age of 18 - and for immunocompromised people from the age of 12. For Johnson & Johnson only has an 18+ license.
In Germany, however, third-party vaccinations from the age of 12 are generally possible without any problems in medical practices and vaccination centers. On December 27, the Federal Ministry of Health assured that the federal government would also guarantee entitlement to benefits for booster vaccinations for 12 to 17-year-olds in the event of vaccination damage. Prerequisite: A mRNA vaccine approved for this group of people was used, i.e. Biontech or Moderna.
By the way: Doctors may also administer vaccines outside of the approval (off-label use). This applies to small children, for example. However, this has an impact on liability in the event of vaccination damage.