And since the beginning of 2010, when the law was enacted, the percentage of Americans aged 19 to 25 without health insurance dipped from 34% to 29%.
The dramatic increase in coverage for a group of Americans that has historically lacked insurance appears to be driven by a single provision in the law that allows young adults to remain on their parents' health plans until they turn 26, according to independent experts such as Paul Fronstin, senior research associate at the Employee Benefit Research Institute.
Almost all of the increase in coverage was in private insurance rather than in public programs such as Medicaid, according to the data from the National Health Interview Survey.
Overall, the share of Americans aged 18 to 64 without insurance dropped slightly from 22.3% in 2010 to 21.3% in 2011.
The expansion in coverage for young adults has cheered the Obama administration and other supporters of the sweeping law, most of whose benefits will not become available until 2014.
"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," Secretary of Health and Human Services Kathleen Sebelius said Wednesday. "Moms and dads around the country can breathe a little easier knowing their children are covered."
Facing persistent public skepticism, the administration has been trying to highlight early benefits of the law such as the young adults coverage and the expansion of aid to seniors who hit the coverage gap in Medicare’s drug benefit known as the doughnut hole.
Last week, the administration reported that 2.65 million people with Medicare coverage have benefited from the additional aid, saving them more than $1.5 billion on their prescriptions.
Allowing young adults, most of whom are healthy, to remain on their parents’ health plans is not as expensive as expanding coverage to other populations with higher medical costs.
But independent analyses have estimated that the expansion could boost premium costs by 1% to 2%.
More ominously, the dramatic rise in the number of young people who are relying on their parents for healthcare may just underscore how bad the economy still is, said Fronstin at the Employee Benefit Research Institute.
"If you have all these young adults who can’t find jobs, they can’t get health coverage on their own," he said. "I don’t know if we would have seen the same effect if unemployment was down below 4%."