(Bloomberg News) The number of people without health insurance fell last year, as many under the age of 26 took advantage of a provision in President Barack Obama's health-care overhaul allowing them to be covered under their parents' plans.

About 540,000 more young people were insured in 2011, helping to keep the proportion of uninsured Americans at about 15.7 percent, the Census Bureau said in a report today. About 48.6 million people were uninsured last year, compared with 49.9 million a year earlier.

The number of uninsured Americans ages 19 to 25 improved to 27.7 percent, or two percentage points fewer than a year earlier, as the 2010 law's mandate for that group took effect. Starting in 2014, the law expands Medicaid, the state-run health program for the poor, and opens new government-run insurance exchanges that may add coverage to about 30 million people.

"A lot of the fall in the uninsured rate and hence the increase in insurance coverage is due to the 19- to 25-year-old age group," David Johnson, chief of the social, economic and housing division at the Census, said on a conference call.

The proportion of people in the country without insurance has risen steadily since the turn of the century, a major impetus for Obama's effort to overhaul the health system. There were 44.8 million people, or 14.9 percent of the population, without insurance in 2008, at the end of President George W. Bush's term, and 36.6 million, or 13.1 percent, uninsured in 2000, at the end of President Bill Clinton's administration, according to Census data.

Obama's administration previously estimated that 3.1 million people who would otherwise lack insurance this year took advantage of the health law to join their parents' coverage.

About 63.9 percent of Americans had private health insurance in 2011, unchanged from 2010, the Census said. It was the first year in a decade that the proportion of people with private insurance didn't fall.