The number of young adults in America who have full-time employment is currently at its lowest in four years, even for those workers who have a college degree, according to a recent Gallup survey.

Less than half, about 44 percent, of adults aged 18 to 29 held a full-time job as of June 2013. That number is down from 46.3 percent in 2010, as well as the previous low of 45.8 percent in 2011. Last year, 47 percent of young adults were employed full-time.

Dennis Jacobe, Gallup's chief economist, attributes the drop in full-time employment for younger workers to a weakened economy.

"The lack of new hiring over the past several years given a recovering economy seems to have disproportionately reduced younger Americans' ability to obtain full-time jobs," Jacobe writes in a report on the survey.

And although young adults who have a college degree are nearly twice as likely to find full-time employment than their peers without a college education, full-time employment for college graduates has also slipped in recent years.

"There appears to be a growing crisis involving a lack of full-time work facing all younger Americans right now, regardless of their education level," the report says.

Currently, about 39 percent of young adults without a college degree and 65 percent of college graduates have full-time jobs. But the rate for college graduates is a 3.5 percent drop from last year, when 68.9 percent had full-time jobs.

But while younger workers are struggling to find full-time employment, older Americans appear to be finding more opportunity. The percentage of middle-aged workers with full-time jobs has remained mostly unchanged since 2010, and that number has actually increased by about 2.5 percent for those aged 50 to 64, to 48.2 percent this year.

Full-time employment percentages for those over 65 remain low in comparison, but there has also been an increase in that age group, up to 8.4 percent in 2013 from 6.2 in 2010.

Although the economy has slowed hiring in recent years, the report suggests that employers may be more willing to hire older and more experienced workers, and that fewer workers are retiring voluntarily.

"This has likely increased the percentage of older Americans with jobs, as companies may be placing a greater value on their experience and productivity and as older workers decide to continue to work when given the opportunity to do so," the report says.

Additionally, the report suggests that these results may be a sign that the workforce will grow more than expected as the economy recovers, when those who "involuntarily left the workplace" return.

And although the "payroll-to-population" rate has seen slight increases and decreases recently, the report says, it has remained "essentially unchanged" in the last year, indicating that there are not enough full-time jobs to keep pace with the growth in population.

"This has had a disproportionately negative impact on millions of younger Americans who are unable to get full-time work," the report says.