The Obama administration on Monday announced that less than a quarter of those who have signed up for Obamacare are between 18 and 34 years old, a figure that falls short of the White House's initial target for ensuring enough younger, healthier Americans enroll in health plans to keep the Affordable Care Act afloat.

With its first public disclosure of Obamacare demographic data, the Department of Health and Human Services said that of the 2.2 million who enrolled in new health plans through federal and state exchanges by Dec. 28, just 24 percent were between the ages of 18 and 34.

The administration trumpeted the findings as proof that Americans, including so-called “Millennials,” were beginning to gravitate towards Obamacare after the botched rollout of the health law’s insurance exchanges.

The White House, however, originally estimated that 2.7 million of a projected 7 million people — or nearly 40 percent — enrolling in Obamacare by the end of March would be from the youngest demographic.

That the administration chose to focus on the youngest Obamacare enrollees Monday shows just how critical the group is to the sustainability of the health care law. For President Obama's signature domestic achievement to work, younger, healthier Americans must enroll in new health plans to keep premiums down for older, sicker patients.

Despite the underwhelming initial returns, the administration on Monday insisted that it was on track for managing the most sweeping overhaul to the health care system since Medicare.

“Among young adults, the momentum was particularly strong,” said Health and Human Services Secretary Kathleen Sebelius during a conference call with reporters.

Republicans, though, said that the numbers showed a lack of enthusiasm for Obamacare among young adults.

“There’s no way to spin it: youth enrollment has been a bust so far,” said Brendan Buck, a spokesman for Speaker John Boehner, R-Ohio. “When they see that Obamacare offers high costs for limited access to doctors – if the enrollment goes through at all – it’s no surprise that young people aren’t rushing to sign up.”

Missing from the administration’s data, however, was the breakdown of those who had actually paid their first month’s premium and the percentage of those who were actually receiving insurance for the first time rather than just shifting plans.

Republicans contend that more Americans lost health insurance than gained it on Jan. 1 because of new Obamacare regulations that forced some insurers to drop existing plans.

Roughly 1.8 million people selected a health plan in December, compared to just 365,000 in October and November, according to HHS. Administration officials also said that 54 percent of enrollees were female and 46 percent were male.

Sixty percent had selected a so-called silver plan and 20 percent had chosen a bronze plan — the most basic coverage option — while 13 percent selected a gold plan and another 7 percent signed up for a platinum plan.

Seventy-nine percent of those signed up for Obamacare exchanges would receive financial assistance, HHS added.

The White House is attempting to reshape the Obamacare narrative after the problem-ridden rollout of diminished the president's standing with the public.

The White House pointed to the Massachusetts health care system, a model for Obamacare, saying that residents there didn't sign up for new plans until the final days before the deadline.

“We think that more and more young people are going to sign up as time goes by, which was the experience in Massachusetts,” said Gary Cohen, deputy administrator and director at the Centers for Medicare & Medicaid Services’ Center for Consumer Information and Insurance Oversight.

The administration is “very pleased with the percentage we have so far,” he added.

This story was published at 4:00 p.m. and has been updated.