The percentage of Americans without health insurance coverage declined to 17.2 percent in the fourth quarter of 2013 from a six-year high of 18.1 percent in the third quarter, according to new estimates by Gallup and Healthways. It's the first time the uninsured rate has decreased since the end of 2012.

The nation's uninsured rate is considerably higher today than it was when Gallup and Healthways began monitoring U.S. adults' health insurance coverage daily in January 2008 as part of the Gallup-Healthways Well-Being Index. The rate was around 14.5 percent in 2008, and it generally increased until the end of 2011, when it was 17.5 percent. It declined throughout 2012, but rose again in the first three quarters of 2013. It now matches the uninsured rate in the second quarter of 2013.

The data was collected in the fourth quarter of 2013, after the health insurance exchanges required under the Affordable Care Act opened on Oct. 1, but before newly purchased health insurance plans became effective on Jan. 1, 2014.

According to data from the U.S. Department of Health and Human Services, younger Americans ages 18-34 made up slightly less than one-quarter of total enrollment through the new insurance marketplaces. That age group makes up approximately 40 percent of the uninsured under-65 population in the country.

The federal government has recruited star power such as basketball legend and HIV survivor Earvin "Magic" Johnson to appeal to uninsured young Americans to sign up for coverage, Bloomberg reported last week.