Obamacare kicked into high-gear in December, with nearly 1.8 million Americans signing up for private health insurance, according to figures released today by the Department of Health and Human Services. But in the latest sign of the law's sputtering start, the figures remain short of the administration's enrollment targets, particularly among young people.

From Oct. 1 through Dec. 28, nearly a quarter of the nearly 2.2 million individuals who selected a health care plan through the state and federal exchanges were between the ages of 18 and 34, Health and Human Services said. Health industry experts have estimated that around 40 percent of enrollees must be young adults for the president's signature health care plan to keep premiums in check.

In the District of Columbia, 44 percent of individuals who selected health care plans were between the ages of 18 and 34, but no state reached the 40 percent figure for youth sign-ups. Arizona, Arkansas, Maine, New Mexico and West Virginia were among the states with the lowest percentage of young people - between 17 and 18 percent - signing up for health care.

The administration said the demographics released today were "similar" to their expectations but said it anticipated more young people would sign up for health care plans closer to the March 31 deadline.

"We think that more and more young people are going to sign up as time goes by, which was the experience in Massachusetts and what we expect to see here," Gary Cohen, deputy administrator and director at the Center for Consumer Information and Insurance Oversight at the Center for Medicare and Medicaid Services, said in a conference call Monday. "We are actually very pleased with the percentage that we have so far, and we expect that percentage to increase."

But the latest figures show that the Obama administration continues to lag behind its enrollment targets. Health and Human Services projected, based on a Congressional Budget Office analysis, that 3.3 million Americans would purchase 2014 health plans through the exchanges through December. It appears that sign-ups are more than a million behind pace.

Still, health industry analysts, including the Kaiser Family Foundation, said the surge in sign-ups in December suggests that reaching CBO's projection of 7 million enrollees by March 31 is not out of the question.

Critical to meeting that target is resolving the pool of Americans who have completed applications but have not yet selected a plan. Health and Human Services says 4.3 million applications were submitted between October and December, but only 2.1 million individuals selected a plan. That means 2.2 million people have yet to lock in their coverage selection. Some are in a state of limbo because of lingering technical problems, others simply because they are still weighing their options.

The state-run marketplaces appear to have had more success in signing people up than the federally run exchange, which covers 36 states. Fourteen states and the District of Columbia run their own exchanges and signed up a total of 950,000 people, close to the 1.1 million signed up in 36 states through Healthcare.gov.

The silver plan was the most popular, with 60 percent of individuals selecting that tier of coverage. The majority of individuals signing up for health care coverage through the marketplace were women - 54 percent. Of the nearly 2.2 million who selected insurance plans, 79 percent received some form of financial assistance.

The next enrollment deadline for health care coverage to kick in on Feb. 1 is Jan. 15 .