The Gallup-Healthways Well-Being Index found the uninsured rate for those ages 18-25 first began to decline in the fourth quarter of 2010 after a provision of the Affordable Care Act went into effect, allowing parents to retain children on their health insurance until age 26.
In 2010, 28 percent of this age group was uninsured, Gallup said.
During the past four years, those ages 26-64 were more likely to be uninsured, with the percentage without health insurance at 19.6 percent up from 15 percent in January 2008.
The percentage of seniors who are uninsured changed little since 2008, varying between 2.8 percent and 3.7 percent. Since seniors qualify for U.S. Medicare, very few adults age 65 and older lack health insurance.
Fewer U.S. adults -- 44 percent in the second quarter of 2012, down from the high of 49.8 percent in January 2008 -- report getting their health insurance through an employer.
The percentage who reported they got their health coverage through Medicare, Medicaid or military/veterans' benefits increased over time and is now at 26.1 percent, the highest Gallup has recorded.
The telephone survey of 57,645 adults was conducted April 1-May 31. It has a margin of error of 2 percentage points.