While this is true, it looks entirely different when looking into the whole context of the vaccination campaign. I am a student in Germany and like many others currently enrolled in universities or for those still in school, politics didn't seem to care at all about us. The official STIKO recommendation nowadays (after they changed it twice) is that it should only be given to people older than 60 (or after consultation with ones general practitioner and if he agrees; in my experience, most don't for young patients).
After young people got (imho rightfully) outraged by a variety of factors, Minister of Health Jens Spahn suggested that young people who did not have the possibility to get an mRNA shot at the time, should get vaccinated with the AstraZeneca vaccine on their own risk and against the official STIKO recommendation.
So taking into account that government officials recommended millions of young people to get the vaccine at their own risk, Merkel (and others!) just "following STIKO guidelines" seems like a weak justification.