It's pretty easy. Smoking never really stopped in Germany. Vending machines are everywhere. They require age verification, which is not hard to bypass.
We got rid of indoor smoking, but we still accept that people smoke.
18 is good. In my previous neighbourhood I lived next to a school. A bunch of 14 year olds with smoke under the supervision of their teachers just outside the gates during recess.
There's no culture of smoking suppression in Germany. You'll see behaviours treated as normal that would trigger an emergency meeting elsewhere.
its pretty common to have school kids 15+ start smoking. and it's easy for them. cigarettes are presented in vending machines everywhere or sold by unbothered vendors.