I've been to Vienna, and the public transportation there is great. I was 12 when I first went there to visit family, and I went everywhere by myself (not 9 as

I've been to Vienna, and the public transportation there is great. I was 12 when I first went there to visit family, and I went everywhere by myself (not 9 as the original, but still younger than the US parents would like). As a tourist I found the way that the destinations were announced over speakers extremely useful, and every stop had maps of the public transportation, timetables, etc. I'd say it's very likely a 9 year old local would have no problem whatsoever, and I wouldn't find it irresponsible (even less so in today's cell phone age).