(CBS/AP) Since the new health care overhaul law took effect a year ago, 2.5 million young adults have gained insurance, according to a new report from the Obama administration.

The report - published Wednesday by the U.S. Department of Health and Human Services - shows an increase 2 1/2 times larger than government and private estimates from earlier this year. The previous estimate was that 1 million Americans ages 19-25 had gained coverage.

Administration officials said they now have more data and are slicing the numbers more precisely than the government usually does in an effort to pinpoint the new law's impact.

Under the health overhaul, children can remain on their parents' health insurance plans until they turn 26. Families have flocked to sign up young adults making the transition to work in tough economic times.

According to the government survey, nearly 36 percent of Americans ages 19-25 - more than 10.5 million people - were uninsured in the third quarter of 2010, before the law's provision took effect. Most workplace health plans started carrying the provision Jan. 1, 2011. By the second quarter of 2011, the proportion of uninsured young adults had dropped to a little over 27 percent, or about 8 million people.
The 2.5 million difference can only be the result of the health care law, administration officials said, because the number covered by public programs like Medicaid went down slightly.

But the health care law's main push to cover the uninsured doesn't come until 2014. Republic presidential candidates are vowing to repeal President Barack Obama's provision, and the Supreme Court is scheduled to hear a constitutional challenge next year.

"The increase in coverage among 19- to 25-year-olds can be directly attributed to the Affordable Care Act's new dependent coverage provision," said a draft report from the Health and Human Services Department obtained by the Associated Press. "Initial gains from this policy have continued to grow as ... students graduate from high school and college."

Who are these young adults?

Some are making the switch from school to work. Others are holding down low-wage jobs that don't usually come with health care. And some - termed the "invincibles" - pass up job-based health insurance because they don't think they'll use it and would rather get extra money in their paychecks.