Why are so many people in their 20s taking so long to grow up?

Why are so many people in their 20s taking so long to grow up? This question pops up everywhere, underlying concerns about “failure to launch” and “boomerang kids.” Two new sitcoms feature grown children moving back in with their parents — “$#*! My Dad Says,” starring William Shatner as a divorced curmudgeon whose 20-something son can’t make it on his own as a blogger, and “Big Lake,” in which a financial whiz kid loses his Wall Street job and moves back home to rural Pennsylvania.

A cover of The New Yorker last spring picked up on the zeitgeist: a young man hangs up his new Ph.D. in his boyhood bedroom, the cardboard box at his feet signaling his plans to move back home now that he’s officially overqualified for a job. In the doorway stand his parents, their expressions a mix of resignation, worry, annoyance and perplexity: how exactly did this happen?

It’s happening all over, in all sorts of families, not just young people moving back home but also young people taking longer to reach adulthood overall. It’s a development that predates the current economic doldrums, and no one knows yet what the impact will be — on the prospects of the young men and women; on the parents on whom so many of them depend; on society, built on the expectation of an orderly progression in which kids finish school, grow up, start careers, make a family and eventually retire to live on pensions supported by the next crop of kids who finish school, grow up, start careers, make a family and on and on.