Spike in US young health coverage
Nearly one million young adults in the US have gained health insurance within the past year, two surveys have shown.
Overall, though, the numbers of people without health insurance increased. Until now, 18-to-25-year-olds have been the most likely to be uninsured.
Healthcare policy is an emotive issue in the US, with Republicans strongly opposed to President Barack Obama's 2010 healthcare law.
Under the law, adults aged under 26 can now be covered by their parents' plans.
The age-specific provision went into effect in September 2010, but many insurance companies implemented the change earlier.
The surveys were carried out by the Centers for Disease Control and by Gallup, a polling organisation.
Neither survey could explicitly link the change in the law to the drop in uninsured young adults.
However, stagnating unemployment numbers and a rise in the overall number of uninsured people suggests the law has had an effect in the 18-to-25 age bracket, correspondents say.
No other age group saw a drop in the uninsured.
Gallup's survey found that 26-to-64-year-olds without insurance rose from 18% to 20% in the same time period, while adults 65 and older stayed at 3.2%.
The Affordable Care Act was passed amid intense political argument in March 2010, and remains a controversial piece of legislation.
It aims to extend health insurance to nearly all Americans, with new taxes imposed on the wealthy and restrictive insurance practices outlawed, such as refusing to cover people with pre-existing medical conditions.
Republican presidential hopefuls have called for its repeal, branding it "Obamacare".
Republicans in the House of Representatives took a symbolic vote to repeal the law in January 2011.
"It's very disappointing to hear some people in Congress talk about repealing the law and taking away this security," said health and human services Secretary Kathleen Sebelius after announcing the CDC's survey.