Young people you say?
Well, in shounen anime, most of the time you'll have 14-15 yo characters with older looks, say, like 18-22, so its completely unreliable.
However, in many cases, you can barely guess at the age.
You can see 16ish looking university students in anime living alone, and its true, many university students start to live alone pretty much in any country, the problem is that they're probably not 16 yo.
I don't know how's it in Japan, but certainly, around 19-20 a person can take care of himself. Although, I must repeat, anime can fool you.