The number of Mainers signing up for health coverage under the Affordable Care Act skyrocketed last month, and most are middle-aged or older, according to new federal figures released Monday that for the first time include demographic data key to the law’s insurance overhaul.

Since the launch of healthcare.gov on Oct. 1, 2013, through Dec. 28, 2013, 13,704 Maine residents selected a health plan through the federal government’s gateway for the marketplaces in Maine and 35 other states, according to a report from the U.S. Department of Health and Human Services. That’s an eightfold increase compared to the month of November.

Enrollment also shot up nationally, with nearly 2.2 million people choosing plans through both healthcare.gov and state-run health insurance marketplaces. December alone accounted for nearly 1.8 million enrollees.

Also called “exchanges,” the marketplaces are geared toward small businesses and individuals who buy their own health insurance rather than receive coverage through work or government programs such as Medicaid and Medicare. In Maine, about 65,000 to 104,000 people are estimated to be eligible to shop on the marketplaces.

In a press release announcing the figures, HHS touted an eightfold increase in the number of young adults ages 18 to 34 signing up for coverage. The Affordable Care Act’s overhaul of the insurance market for those who buy their own coverage relies on young, healthy Americans signing up to help share the risk with older, sicker consumers and keep costs from skyrocketing.

Americans aged 18 to 34 accounted for nearly a quarter of the 2.2 million people who have selected health plans so far. That’s short of the White House’s target, but administration officials stressed that young people still have plenty of time to enroll.

“We expected older adults to sign up early and we expect more young adults to come in [during] the month of March, by the end of the open enrollment period,” Nancy Delew, HHS’ acting deputy assistant secretary for planning and evaluation, said on a press call Monday.

The White House previously said that if 7 million people ultimately enroll in coverage as projected, 2.7 million, or 40 percent, should be young adults whose premiums would subsidize the costs of older, less healthy enrollees.

In Maine, the oldest state in the country, the proportion was lower, with 18 percent of enrollees under age 35. Only two states, Arizona and West Virginia, posted lower showings at 17 percent. Maine tied with New Mexico and Arkansas. Data was not available for three states.

More than 60 percent of those who had signed up for coverage in Maine were between the ages of 45 and 64.

“I want to see more people of every age enroll. I don’t think these numbers for Maine versus the national age numbers are particularly surprising or of concern at this stage,” said Mitchell Stein, policy director for the advocacy group Consumers for Affordable Health Care.

Many health policy experts expect younger, healthier adults to put off buying coverage until open enrollment winds down on March 31. Older, sicker consumers are more likely to already have coverage that they want to continue, or to jump at the chance to buy it, while younger adults tend to lack coverage and may wait to purchase a health plan.

Kevin Lewis, CEO of Maine Community Health Options in Lewiston, one of two insurers selling health plans on the marketplace in Maine, said while he expects more young people to sign up in the coming weeks, not all older people are necessarily sicker.

“In general, the older we get the more we need to utilize health care services, that’s a truism,” he said. “But I think there’s broad diversity within any given age.”

The new report also showed that 87 percent of the nearly 14,000 Mainers who have chosen a health plan through Healthcare.gov got financial help to afford their coverage.

In Maine, 72 percent opted for a “silver level” plan, which is linked to a discount that lowers the amount policyholders pay out of pocket. Policies range from platinum to gold, silver and bronze, with the higher metal tiers carrying heftier monthly premiums but lower co-pays and deductibles.

Tax credits also are available to lower consumers’ monthly premiums.

At Maine Community Health Options, which also sells plans off the marketplace, roughly 10 percent of all enrollees have signed up directly with the nonprofit insurer, bypassing H ealthcare.gov, Lewis said.

Monday’s report also showed that 55 percent of Maine consumers who chose a plan through Healthcare.gov were women.

The enrollment numbers reflect consumers who have chosen a plan but may not have paid their first premium, which is how insurers typically define enrollment.

Stein cautioned against reading too heavily into the numbers, with half of the open enrollment period still to go.

“This represents millions of people having health coverage and having that security and most of them probably didn’t have it beforehand,” he said.

Affordable Care Act 101

We break down the ACA, what it does and what it requires you to do.

• Just the FAQs ma’am: Frequently asked questions and helpful answers
• An ACA Timeline: How health care coverage has evolved.
• Where to get assistance and more info: There are more sources of information and help understanding how the Affordable Care Act affects you.