All college-educated young adults who live at home with their parents should read Neal Templin's "Learning Life's Lessons On a Shoestring Budget," WSJ, August 28, 2008; Page D2. For now, the article is posted at the link below, but no permalink exists. If the link below does not go to the article, try googling "Learning Life's Lessons On a Shoestring Budget."
It wasn't always a given that kids would move out when they reached adulthood. Children used to live at home until they married, and it's still that way in many other countries.
This arrangement also allows parents to have more sway over who their daughter or son marries, thereby providing more leverage for their expectation of grandchildren. For more on this subject, 60 Minutes did an entertaining report on Italy's men this year, called "Mammoni."