After at least five years of media hype warning that a tectonic societal shift was slowly taking place, it has hit home. Millions of parents who used to worry vaguely about what they'd do when their kids fled the nest are now fretting about the opposite: how to get them to leave. An estimated 18 million fledgling adults are now out of college but not out on their own. Parental nests are packed with offspring whose costly college educations so far have not equipped them to assume the traditional markers of adulthood: moving out on their own, finding jobs good...