The federal health-care reform law increased the age that individuals can stay on their parents' insurance plan to 26, a U.S. Department of Health and Human Services (HHS) press release noted. That policy took effect in September 2010. Between then and June 2011, the percentage of adults between the ages of 19 and 25 with private insurance has increased significantly from 64 percent to 73 percent, according to HHS, leading the CDC to reach its 2.5 million estimate.
The number of adults from ages 26 to 35 with health insurance has remained stable at 72 percent, according to a HHS issue brief, which "makes it clear that the increase in coverage" for ages 19 to 25 is attributable to the ACA.
“Thanks to the Affordable Care Act, 2.5 million more young adults don’t have to live with the fear and uncertainty of going without health insurance,” HHS Secretary Kathleen Sebelius said in a statement. “Moms and dads around the country can breathe a little easier knowing their children are covered.”
The data was drawn from the CDC's National Health Interview Survey.