Age discrimination is illegal: https://www.gesetze-im-internet.de/agg/__1.html
You can sue the landlord for compensation, which is usually a few thousand euros plus the fees for your lawyer. You cannot sue the landlord for renting you the apartment.
If you want to sue the landlord, you should save the ad, contact the landlord and apply for the apartment. You can only sue if the landlord refuses to give the apartment to you. The text of the ad proves that illegal age discrimination is the motivation for the refusal. Contact any local lawyer for tenants law (Fachanwalt für Mietrecht).