Just 24 percent of Americans signing up for coverage under President Obama's health care law through December were part of the young adult demographic, the Department of Health and Human Services announced Monday -- well below the nearly 39 percent the White House had once deemed essential to the law's success.

In a new report, HHS said that through Dec. 28 (about halfway through the six-month open enrollment period), roughly 2.2 million Americans signed up for coverage on one of the law's health insurance exchanges. That's well below the administration's target of 3.3 million. And HHS still hasn't disclosed how many of those who have signed up for insurance have actually paid their first premiums, which is necessary for enrollment to be finalized.

Leading up to the launch of the exchanges Oct. 1, administration officials had been telling reporters that in order to be a success, 2.7 million of the projected 7 million enrollees in the health care law's exchanges would need to be from the young adult demographic. Attracting a critical mass of young and healthy enrollees was seen as necessary to offset the cost of covering older and sicker Americans, particularly those with pre-existing conditions. But in the report, HHS said that just 24 percent of those who signed up so far were aged 18 to 34.

In December, a report from the Kaiser Family Foundation identified a “worst-case scenario” situation in which just 25 percent of enrollees were in the 18-to-34 demographic.

In a conference call held earlier Monday and embargoed for release at 4 p.m. EST, Nancy Delew, acting deputy assistant secretary for planning and evaluation at HHS, insisted that the number of young adults who had enrolled so far was “similar to our expectations” and proportional to the percentage they represent of the under-65 population.

Along with other officials on the call, Delew emphasized that the administration expected younger people with lower medical costs to sign up later in the process. The open enrollment period lasts until March 31, after which point Americans will be subject to the individual mandate penalty for failing to obtain health insurance.

Though there’s a plausible case to be made that younger Americans will wait until later to sign up, the administration is still in a deep hole. Because the current number of young adult signups is significantly less than 40 percent, to make up ground, signups in the coming months will have to be significantly higher than 40 percent.

As an example, let’s just say all of the roughly 2.2 million Americans whom HHS says have signed up for insurance pay their premiums and complete enrollment, and the total paid enrollment number through March ends up being 5 million people. To meet the original demographic goal, about 1.4 million of the remaining 2.8 million enrollees — or roughly half — would have to be between the ages of 18 and 34.

That said, ultimately, it isn't the overall national figure that matters. What's important is that the law creates functioning exchanges in all 50 states, plus the District of Columbia. Right now, some states are doing better than others. For instance, in Maine, West Virginia and Arizona, just 17 percent of enrollees came from the key age demographic. On the other hand, in D.C. -- which received a boost from young Capitol Hill staffers who were forced onto the exchange -- 43 percent of those signing up were in the target age group.

It's worth stipulating that age is only a rough approximation of the health status of the overall risk pool, as it's possible to have young and sick enrollees or older and healthier enrollees.