(Bloomberg) -- How come more people are retiring in their early 20s?

Why are middle-age men becoming stay-at-home dads?

What's keeping women out of the workforce other thanillness, kids or school?

Those are some of the questions raised in a new Bureau of LaborStatistics reportthat shows changes over the past decade in whypeople stay out of the labor force.

Read: Age discrimination is widespread in jobmarket

Finding answers is key for the Federal Reserve asit maps the contours of a job market that's becomingharder to predict with the aging of the baby boomers andshifting household priorities.

Here's what the bureau found, broadly: Thirty-five percentof the U.S. population wasn't in the labor force in 2014, upfrom 31.3 percent a decade earlier. (You're considered out of theworkforce if you don't have a job and aren't looking for one.That's distinct from the official unemployment rate, whichtracks those out of work who are actively job hunting.)

Drilling down into the numbers reveals more about the shifts inthe reasons some people forego a paycheck. In all age groups,for instance, more people cited retirement as the reason for beingout of the labor force, and it wasn't just older people.

For Americans between the ages of 20 and 24, the share ofthose sidelined over the past decade because they were in schoolincreased, unsurprisingly, during the decade that included theGreat Recession.

What's more unusual is that the share of 20- to 24-year-olds whosay they're retired doubled from 2004 to 2014.

Other reasons for not working are also on the rise. More menbetween 25 and 54 cited home responsibilities, while women of thesame age range increasingly point to illness or school as theleading cause. The data also show, perhaps not surprisingly, thatmen and women without a high-school diploma are more thanthree times as likely to be out of the workforce than their peerswith a college degree.

Demographic changes aren't the only pieces of the puzzle thathave changed the employment landscape since 2004. The BLS reportshowed that among male veterans between 25 to 54 years old, thenumber who reported a service-connected disability rose to 1.2million in 2014, from 726,000 in 2003. That rise coincides withU.S. military combat in Iraq and Afghanistan.

Fewer people willing or able to take a job might eventuallycause a shortage of workers, leading to surge in wages andlonger-term inflationary pressures, according to PrincetonUniversity economist Alan Blinder, a former Fed vice chairman.

"We're going to be running out of labor as we go throughtime,'' Blinder saidon Bloomberg Television Dec. 31.

Understanding why people aren't in the workforce--andwhether it's permanent or temporary--is important for theFed.

Policy makers are trying to estimate remaining slack in thelabor market and the outlook for inflation as they weigh the timingof the next interest-rate increase.

But as Chair Janet Yellen told a press conference on Dec. 16,after the central bank lifted rates for the first time since 2006,it may be a while before Fed policy has to contend with an actuallabor shortage.

"The labor-force participation rate is still below estimates ofits demographic trend, involuntary part-time employment remainssomewhat elevated, and wage growth has yet to show a sustainedpickup,'' she said.

Officials will get more information when the Labor Departmentreleases December's payroll report on Friday. Economists surveyedby Bloomberg News expect that employers added 200,000 last month,compared with 211,000 in November, while unemployment rate probablystayed at 5 percent.

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