There was a German social network a few years ago that did exactly that (before Facebook was available in German)

They had SchülerVZ for kids/teens, then they had StudiVZ for university students and finally they had MeinVZ for adults. The problem was, that they weren’t interconnected at all apart from the option to move your account to the next platform. So if you were just starting to study but you still had friends that were in school, you could’t keep in touch with them.

18% was before christmas. Now they are at 20-21% in the polls.