https://www.bvg.de/en/tickets-tariffs/conditions-of-carriage
https://www.bvg.de/en/tickets-tariffs/conditions-of-carriage
> Children under four years of age may only use public transportation if they are accompanied by an individual who is at least six years of age.
(edit: unfortunately not, for in the German it is more specific, "nur in Begleitung einer Person …".)
- actually navigate to the intended destination
- Communicate and find help from adults in the case of any issues
I used to live across the street from a primary school and the kids (no older than 10) would come out and smoke at recess in front of my building. Nobody batted an eye. Just yesterday I was walking behind some kids who couldn't have been older than 12 drinking and smoking, and that was in a nicer Berlin "suburb". Nobody cared.