Even in urban areas it was not uncommon for young adults to live with their parents (and grandparents) until they married, at least up until the end of WWII. W

Even in urban areas it was not uncommon for young adults to live with their parents (and grandparents) until they married, at least up until the end of WWII. Women wouldn't move out because it was considered improper for an unmarried woman to live away from her parents; and men wouldn't move out because they wouldn't have anyone to cook and clean and launder for them, and most didn't have the skills to do this themselves (though some would move to lodgings where a certain amount of this was taken care of for them). Suburbanization and the increased availability of housing helped change that. But I'd guess the biggest contributor would have been the increasing age at first marriage. Living with your parents until marriage is one thing if you're married by 21, and quite another if you're single at 30 and can afford to move out - which was increasingly the case in the second half of the 20th century. Of course, now fewer and fewer can afford to move out, even if they're 30+ and unmarried.