Quote: "Urlaub im Ausland sollten Jugendliche erst ab etwa 16 Jahren allein oder mit Gleichaltrigen machen dürfen. Und dann auch nur mit der nötigen Vorbereitung. Ideal ist es, wenn vorher schon mal in der Schweiz «geübt» worden ist. "
I found this here:
http://osz.gommiswald.ch/uploads/med...BCr_Eltern.pdf
Translation: "Teenager should be approx. 16 yrs old if they want to go on holidays on their own or with peers. They should only go well prepared. It would be good if they earned some practise in travelling on their own beforehand in Switzerland.
I've been traveling on my own within Europe at age 16/17, that was ok. I don't know if there exists a law but I'd say no. For taking the bus or train there is no age limit, it depends on you as parents what you'll allow. As a mother of a 6,5 year old girl, I'd say that she'll be ready to take the bus on her own approx. in 2 yrs - as soon as she'll be fluent in reading. But this will only be on short distances without changing bus.