Peter G. Spitzer

You say your son graduated from Penn in 2008 and is still planted in his boyhood bedroom? Your daughter with the master’s degree spends her days on your living room sofa, watching “Jersey Shore” reruns? Meet the guy who’s given them official permission not to grow up: Temple developmental psychologist Laurence Steinberg.

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Way back in 1990, Steinberg penned a parent-help book, You and Your Adolescent: A Parent’s Guide for Ages 10-20. A revised edition in 1997 had the same title, but the newest version, which just came out, extended the “adolescent” label through age 25.

Have young people changed so much in 14 short years?

Not so much the kids as the science, says Steinberg. Advances in brain imaging have let neuroscientists study adolescents much more closely — and determine that the prefrontal cortex, which houses complex decision-making, isn’t fully developed until the mid-20s. At the same time, the recession has more kids holed up in Mom’s basement playing Xbox: “Clearly, it’s harder in this economy to transition to adulthood — to get a job, pay for a residence, get married,” the professor says.

But don’t just blame the kids. Steinberg says colleges everywhere are “increasingly perplexed by the degree of involvement on the part of today’s parents" — those hovering helicopters who call up profs and complain about Connor’s biochem grade or Alyssa’s roommate. Just because your child is economically dependent, Steinberg says, doesn’t mean you should have the same level of emotional codependence as when she was 13.

It’s hard, though, to let go when your kid is eating supper at your kitchen table and washing her bras in your sink. Steinberg’s advice? Make sure children have chances to make independent decisions even if they’re still under your roof, and help them seek out guidance from grown-ups other than you.

It’s also hard not to resent a kid’s continuing presence when you had a completely different vision of how you’d live once the college years were done. Steinberg has heard parents complain: “We love our kids, but we were planning on saving for retirement.” One mom we know whose three grown sons all still live at home has taken to going away for weekends with her husband, just to get a little space.

And when adult children stay home, boundaries have to be negotiated all over again: Who makes the rules, and until what age do they apply?

“If I’m supporting a child,” says Steinberg, “I have the right to comment on how she’s living her life. When we do that with 16-year-olds, no one’s surprised.” When your offspring’s 24, your commentary’s going to be far less welcome — yet the issues are still there.

Steinberg didn’t move the milestones; he just made the changes official. And it’s unfair, he says, for parents to judge a child’s progress against what they were doing at the same age when they’re unwilling to relinquish financial and emotional control. He even sees a potential silver lining to the adolescent stretch: If kids aren’t rushing into life decisions — where to buy a home, what career to choose, whom to marry — they might make wiser choices. And that could lead to happier children, which, in the end, is what all parents want.