I have gone to live with my parents twice in adulthood, both very short term (for example, the first time, my condo lease had just expired and I expected to be moving out of state within a couple of months, so didn't want to sign a new lease), and I paid room and board both times.
Your situation is a bit different, though. If my mother and I were both single and would otherwise live alone, I wouldn't mind at all renting or buying a house with her. We get along great, we know each other's quirks, we enjoy many of the same hobbies and pastimes, and those that we don't share wouldn't conflict with each other's. It would be a fine situation for us.
I can completely understand anyone who would NOT want to do something like that, however.