The Altstadt is socially tricky. Central and pretty, yes. But also faster to tip into spending pressure and aimless wandering. That is why the district only becomes socially strong once the plan has a clear frame.
What works here
1. One cultural or spatial anchor
Haus der Natur, one clear route or one official place often works much better than simply "going into town".
2. Culture context instead of shopping mode
A museum, one route with a view or another official place lowers pressure far more than a vague center meetup.
3. Enough staying quality for real time
The best youth plans here should hold at least 60 to 180 minutes without constant buying or improvising.
Conclusion
The Altstadt becomes useful for teen connection when the plan is clear, local and low-friction instead of relying on city-center randomness.