WASHINGTON - Less than three weeks remain in the open enrollment period for the healthcare insurance marketplace commonly known as Obamacare, but according to new statistics from the administration today a critical age group still isn't turning out in needed numbers.

As of March 1 over 4.2 million have found insurance coverage through the online state and federal exchanges mandated by the Affordable Care Act, according to the Department of Health and Human Services. It's a 22 percent increase over January's 3.3 million and within possible striking distance of the 6 million estimated by the non-partisan Congressional Budget Office to enroll before the March 31 deadline.

But persons aged 18-34 represent only a quarter of the enrolled, down two points from a month ago. It is a crucial missing demographic for the law, which banks on the young and healthy Millennial generation to bring down costs. The White House has previously asserted that if enrollment reached 7 million, Millennials would need to make up 40 percent to keep insurance prices from rising.

Regardless, the administration maintains they expect millions more Americans - particularly youth - to sign up before the end of the month, owing to last-minute procrastinators.

"We learned from the Massachusetts experience that young adults tend to sign up later in the process. So we know that millions of Americans are finally obtaining the security of affordable health coverage and that's there still time for others to sign up," HHS Secretary Kathleen Sebelius said in a conference call with reporters today, referring to the state's similar healthcare overhaul in 2007.

And the White House and its allies have lobbied hard for that demographic through celebrity endorsements, humorous ads, social media campaigns, and other targeted outreach programs. Just hours before the HHS report, President Obama himself pitched his signature achievement in an interview with actor and comedian Zach Galifianakis for a "Funny or Die" video web series.

Previous ads have ranged from a guitar-strumming hipster via Oregon's state-run exchange, to a young man in pajamas , care of the president's independent advocacy group, Organizing for Action.

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Republicans immediately jumped on the numbers today as a canary in the coal mine.

"It seems the president's push to enroll young adults is far too little, too late," House Speaker John Boehner's office said in a written statement. "The administration won't tell us how many people have actually paid for a plan or how many were previously uninsured. But what we do know is that young adults - those who the White House repeatedly said are critical - are deciding the health care law is a bad deal."

People who are not covered by health insurance by March 31 may be hit with fines, although a wide range of exemptions exist for reasons including hardship and incarceration. Eighty-three percent of enrolled have selected plans eligible for financial assistance, according to the administration.

This story was updated at 5:23 PM.