The total U.S. population age 16 and over is at least 243 million. Subtracting the nearly 156 million Americans in the labor force in June 2013 -- that is, those who were either working or looking for work -- leaves 87 million Americans, which is close to 90 million.
However, the 90 million number is padded, since this number includes a lot of Americans who wouldn’t be expected to be working. Specifically:
• People age 16 to 17, who likely are in high school: 9 million
• People who are enrolled in either two- or four-year colleges: 21 million
• People age 65 and older, who have reached retirement age: 40 million people
That means 20 million people are of normal working age, not in college and not working. That’s less than one-quarter the amount repeatedly cited in the blogosphere.
So the 90 million number is exaggerated. Even so, the idea that fewer people are joining the workforce is something that worries economists.
All things being equal, economists like to see more people working because it helps economic growth (though not at the expense of dropping out of school, which can limit future employment opportunities and earnings potential).
To gauge these trends, economists can calculate the labor force participation rate, which is the percentage of the population that is either working or looking for work, and they can calculate the employment to population ratio, which is the percentage of the 16-and-over population that is currently employed. Both statistics generally track each other, but not in lockstep.
https://www.politifact.com/truth-o-meter/statements/2013/jul/30/blog-posting/are-90-million-americans-not-working-or-looking-wo/