This depends largely on culture and where and how you are raised. I think with most foreign families, you live with your parents until you get married. Its usually to save money and be able to buy a house. The parents never charge the kids rent.

Some families in North America I heard that they will start to charge their kids rent when they turn 18. Maybe its to convince them to leave and get a place of their own or its to maybe finally pay back all the money that the parents spent on them. Either way, with housing being so expensive these days, many kids have no choice but to live with their parents, even if it means paying a little rent.