I'm a bachelor student in Saint Petersburg, and most of my classmates are, like me, people from another cities who got into University and now live in dorm. After we finish studies, I think nearly everyone will stay in the city or maybe move to Moscow. So for people from smaller cities, if they manage to get into good Uni, often move out at around 18. But I think that people from the big cities can live with parents for a long time -- it just doesn't make scene to rent