WASHINGTON — The share of young adults without health insurance fell by one-sixth in 2011 from the previous year, the largest annual decline for any age group since the Centers for Disease Control and Prevention began collecting the data in 1997, according to a new report released on Monday.

The estimates are drawn from a federal survey of about 35,000 households. It did not ask how the newly insured obtained coverage, but the study’s author, Matthew Broaddus, a research analyst at the liberal Center on Budget and Policy Priorities, said the increased coverage for young people was almost certainly due to a provision in the Obama administration’s Affordable Care Act that allows children to stay on their parents’ insurance policies until their 26th birthday.

Joseph Antos, a health care policy expert at the conservative American Enterprise Institute, agreed that the provision of the new law was the only plausible explanation for the increase. He pointed out that young people have been among the hardest hit in the recession and would otherwise have been expected to be less likely to be insured. “Nothing else went well for this age group,” he said.

The share of people ages 19 to 25 who lacked health insurance fell to 27.9 percent, down from 33.9 percent in 2010, or about 1.6 million fewer uninsured people, according to the data from the federal study, known as the National Health Interview Survey. For the next age group — those 26 to 35 years old — the share of the uninsured rose, a further sign, Mr. Broaddus said, that the health care law was driving the decline among the younger group.