The economic success of the Affordable Care Act depends on young, healthy people to enroll. The Post's Karen Tumulty takes a look at some of the states' most controversial ads selling Obamacare. (Julie Percha/The Washington Post)

The economic success of the Affordable Care Act depends on young, healthy people to enroll. The Post's Karen Tumulty takes a look at some of the states' most controversial ads selling Obamacare. (Julie Percha/The Washington Post)

Young adults account for slightly less than one-fourth of the Americans who signed up for health plans during the initial three months of federal and state insurance marketplaces — fewer so far than the government has said will be needed to make the economics of the new exchanges work.

The figures, part of a monthly progress report on the marketplaces that was issued Monday, offer the first glimpse into whether the health plans available under the Affordable Care Act are becoming provinces of the old and sick or are managing to attract young, healthy people who have not previously considered insurance worthwhile.

According to the report, released by Health and Human Services Secretary Kathleen Sebelius, 24 percent of the nearly 2.2 million people who enrolled in the marketplaces through the end of December are between the ages of 18 and 34. One-third are 55 to 64 years old.

The figures mean that the proportion of young adults is lagging behind what both government and outside health policy analysts have said will be required for the exchanges to remain stable. Analyses have concluded that, to prevent health plans’ premiums from rising and some insurers from potentially dropping out, roughly two in five Americans in the plans should be young adults.

In releasing the report, Obama administration officials were upbeat. The latest enrollment figures are “solid, solid news for us,” a senior administration official told reporters at a White House briefing. The officials predicted that the health plans would attract more young adults during the second half of the open-enrollment period for this year, which continues through March.

Still, the report’s numbers increase the significance of outreach efforts designed to persuade young people to sign up. Whether sufficient numbers of young people will do so, despite the law’s requirement that most Americans have coverage or face a financial penalty, has been an uncertainty hovering over the law.

Administration officials said Monday that their plans to intensify promotional efforts aimed at young adults will focus on two dozen communities in states such as Texas that rely on the federal marketplace and have especially large uninsured populations.

Health policy specialists and industry officials said it is too early to know whether the enrollment patterns will shift in the next few months, although Republicans wielded the new figures as their latest weapon to condemn the federal health-care law.

“Youth enrollment has been a bust so far,” said Brendan Buck, spokesman for House Speaker John A. Boehner (R-Ohio).

Larry Levitt, senior vice president at the Kaiser Family Foundation, said the proportion of young people “is lower than would be ideal.” Still, he said, the data were “encouraging,” given that people have another 21 / 2 months to enroll and that the federal online system, HealthCare.gov, had defects that prevented many people from signing up for much of the fall.

In a study last month, the foundation concluded that 40 percent of the people who could benefit from the new insurance marketplaces are 18 to 34 years old — and therefore, the pool of people who join the plans should match that proportion. The study also predicted that if the proportion of young people remains roughly as low as it is now, it could reduce health plans’ income from premiums in ways that, while not huge, could nevertheless cut their typical profits roughly by half.

Robert Zirkelbach, spokesman for America’s Health Insurance Plans, the industry’s main trade group, said the impact of who buys coverage “is more a continuum than a cliff” and pointed out that insurance prices are influenced by both the age of customers and how much health care they need. “The more young, healthy people participating, the more stable the marketplace will be, and the more affordable coverage will be,” he said.

The enrollment report said that, in the three dozen states relying on the federal marketplace, 23 percent of the nearly 1.2 million people who enrolled by late December are ages 18 to 34. In 14 states, plus the District of Columbia, that are running their own new marketplaces, 25 percent of the nearly 900,000 people who enrolled fall within that age group.

Young adults accounted for 44 percent of the people who signed up for health plans in the District, the highest percentage in the country. In both Maryland and Virginia, the share of 18-to-34-year-olds was 27 percent.

The overall enrollment of nearly 2.2 million in Monday’s report, from the marketplaces’ start on Oct. 1 through Dec. 28, echoes information the White House previously issued. The data show a seven-fold upswing in enrollment in the federal exchange from the first two months as the Web site’s ­performance improved. Federal health officials said, however, that they do not yet know how many of those people were uninsured before — or how many of those who enrolled have paid their first month’s premium in order for their coverage to start.

While Americans can sign up through March 31 for coverage in 2014, the December enrollment period was especially significant because the health plans took effect on New Year’s Day and anyone who signed up by Dec. 24 — and pays the first premium by various deadlines this month — is covered from the beginning.

Beyond the age breakdowns, the report issued Monday contained the first information on the sex of people buying new health plans, showing that 53 percent are women and 47 percent are men.

The report also showed that, of four tiers of coverage, named for different metals, by far the most popular are the “silver” plans — the second level from the bottom — which outside health analysts have found have a typical insurance deductible of $2,500, far more than traditional health coverage.

Under the law, the government is for the first time providing subsidies to help Americans pay for private health insurance. During the first three months of enrollment, the report said, about four in five people who signed up qualified for a federal tax credit.