Ever since it became law in March 2010, the Affordable Care Act has been mocked from both sides of the aisle for its slow rollout and intangible benefits. But a key provision that kicked in early — allowing young people to stay on their parents’ health insurance until the age of 26 — has proven wildly popular and is paying big dividends:

Young adults, long the group most likely to be uninsured, are gaining health coverage faster than expected since the 2010 health law began allowing parents to cover them as dependents on family policies.

Three new surveys, including two released on Wednesday, show that adults under 26 made significant and unique gains in insurance coverage in 2010 and the first half of 2011. One of them, by the Centers for Disease Control and Prevention, estimates that in the first quarter of 2011 there were 900,000 fewer uninsured adults in the 19-to-25 age bracket than in 2010.

This was despite deep hardship imposed by the recession, which has left young adults unemployed at nearly double the rate of older Americans, with incomes sliding far faster than the national average.

The Obama administration, intent on showcasing the benefits of a law that has been pilloried by Republicans, attributes the improvement to a provision of the Affordable Care Act that permits parents to cover dependents up to their 26th birthdays. Until that measure took effect one year ago this week, children typically had to roll off their parents’ family policies at 18 or 21 or when they left college.

Some twenty-somethings adopted a posture of “young invincibility,” forgoing health insurance they could afford while gambling that they would not incur steep medical expenses. But others, like Kylie R. Logsdon, who credits the provision for enabling her heart transplant in July, were living with chronic or life-threatening conditions and had no prospects for coverage.

“I honestly don’t know what we would have done,” said Ms. Logsdon, 23, of Gerlaw, Ill., who gained coverage under her father’s policy after losing her job as a legal secretary. “There was no way we could have afforded it. I might not be here right now.”

Last week, the Census Bureau reported that the share of young adults without health insurance dropped in 2010 by 2 percentage points, to 27.2 percent. That decline meant that 502,000 fewer 18- to 24-year-olds were uninsured. Most gained coverage through private policies, not government programs.

For every other age cohort, the proportion without insurance increased, as high unemployment and contractions in employer coverage continued to take their toll. For the first time in more than 10 years, 18- to 24-year-olds were not the least insured group, having been overtaken by those 25 to 34.

Kathleen Sebelius, the secretary of health and human services, accentuated the silver lining in an otherwise grim Census poverty report by declaring: “The Affordable Care Act is working.”

Obama needs a steady trickle of reports like this one to slowly improve impressions of the healthcare law, which remains his signature domestic achievement, before he’s on the ballot again in November 2012.