America's unemployment rate is at a half-century low, but it also has a job-quality problem that affects nearly half the population, with a study finding 44% of U.S. workers are employed in low-wage jobs that pay median annual wages of $18,000.

Contrary to popular opinion, these workers aren't teenagers or young adults just starting their careers, write Martha Ross and Nicole Bateman of the Brookings Institution's Metropolitan Policy Program, which conducted the analysis.

Most of the 53 million Americans working in low-wage jobs are adults in their prime working years, or between about 25 to 54, they noted. Their median hourly wage is $10.22 per hour — that's above the federal minimum wage of $7.25 an hour but well below what's considered the living wage for many regions.

Even though the economy is adding more jobs, there's increasing evidence that many of those new positions don't offer the kind of wages and benefits required to get ahead. A new measure called the Job Quality Index recently found there is now a growing number of low-paying jobs relative to employment with above-average pay.

For the U.S. overall, median household income is $66,465, according to Sentier Research, with roughly half of families earning less than that amount.

Workers aren't shy about expressing their frustrations, with about 6 of 10 workers saying their jobs are mediocre to downright bad, according to a recent Gallup job-quality survey. For instance, 1 in 5 workers told Gallup their benefits are worse now than five years ago.