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.



(Read more: Student-loan rates at 9%: That's a better deal?)

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.

