A record 21.6 million millennials live with Mom and Dad

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More than a third of American millennials are living at home with their parents, according to a Pew Research Center study released Thursday.

When the recession began in 2007, 32 percent or 18.5 million of millennials—defined as 18- to 31-year olds—had not left the nest. Today, it is 36 percent, or 21.6 million.

According to Pew's analysis of Census Bureau data, the number of millennials still living at home is the highest percentage in four decades.

A driving factor: declining employment. Last year just 63 percent of young adults in that age group had a job, down from 70 percent in 2007.

"You're much less likely to be living with your mom and/or dad if you have a job, and job-holding still hasn't picked up," said Richard Fry, a senior research associate with Pew Research Center.

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Another interesting finding in the study is the gender difference. Men are more likely to hang at home than women: 40 percent to 32 percent.

What's a parent to do?

It costs a little less than $300,000 to raise a child to age 17, according to Department of Agriculture figures. And maybe another $160,000 to put him through college. But as the Pew study reveals, more families are finding that the big bills don't stop once kids reach adulthood.

Today's offspring who fail to launch have become a major financial planning issue, said certified financial planner Lynn Ballou, a managing partner at Ballou Plum Wealth Advisors in Lafayette, Calif.

Parents already have a tendency to sacrifice their own retirement planning and other savings to send their children to college, banking on them to find good jobs at graduation.

"Come to find out, your grad is unemployable and can't even find a job at Starbucks," said Ballou, who estimates that one-third of her clients are providing some financial support to an adult child.

Unexpected expenses to care for adult children can force parents to make severe cuts in their own spending, delay retirement or dip into savings.

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But parents shouldn't feel obligated to continue providing the same support as they might for a teenager.

"They could be there forever if you don't charge them some rent and make them do some chores," said certified financial planner Sheryl Garrett, founder of The Garrett Planning Network. Adult children who can't find a job outside the home should be asked to contribute with duties in the household, she said.

If there's good news, it's that boomerang babies aren't entirely those who can't afford to live on their own. By Pew's estimates, one-third to one-half of those young adults living with Mom and Dad are college students who spend most of the year on campus and come home for breaks. (Whew.)

College graduates are also less likely than less-educated counterparts to live at home, particularly after they turn 25, Fry said.

Parents of children college-aged and younger may find it prudent to beef up their emergency fund from the recommended six months' worth of living expenses to 12 months, just in case.

"From where we're sitting, it could be a good decade to work this out," said Ballou. "Hope for the best and plan for the worst. Assume your child is not going to launch until they're in their late 20s."

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Even when the job market picks up, however, parents may find they need to put off remodeling Junior's room into a home office.

"It's not just the poor economy," Fry said. "There indeed may be less stigma among young adults about living at home. Even when the economy fully recovers, the tendency may be to live at home longer."

By CNBC.com's Kelli B. Grant. Follow her on Twitter @KelliGrant.