WASHINGTON — As many as a million young adults have signed up for health insurance in the last year, new data indicate, suggesting the success of an early benefit of the health care law President Barack Obama signed last year.

A survey by the Centers for Disease Control and Prevention found that the number of Americans aged 19 to 25 without insurance fell from 10 million in 2010 to 9.1 million in the first three months of 2011.

And a Gallup-Healthways poll showed that the rate of uninsured adults aged 18 to 25 fell from 28 percent last fall to 24.2 percent in the second quarter of this year.

Starting last September, the new health law began allowing adult children up to age 26 to remain on their parents’ health plans.
“The provision … appears to be having an immediate effect on the number of Americans who report they have health insurance,” Gallup concluded in a note accompanying its survey.

Americans in their late teens and early 20s remain the most likely to go without insurance.

By comparison, fewer than 7 percent of children under 18 lacked health insurance, according to CDC survey, reflecting the wide availability of government health programs children from low- and middle-income children, including Medicaid and the Children’s Health Insurance Program, or CHIP.

The new insurance numbers were quickly hailed by the Obama administration and its allies, who have been looking for evidence that the new law is having a positive impact, especially because many of its benefits are still years away.

“This news today demonstrates a great victory for young Americans, and is evidence that the new health care law is working for our generation,” said Jen Mishory, deputy director of Young Invincibles, a supporter of the law.

“In this tough economy, it’s even more important that young adults have access to decent, affordable coverage. Because of dependent coverage, 1 million more of us have that access,” she said.

The uptick in health coverage among young adults was also picked up in recent census data and reinforced by reports from insurers that many Americans were taking advantage of the new health care law’s benefit.