The Obama administration provided more details about the 8 million people who have enrolled in Obamacare by the first major deadline, for the first time reporting concrete age and demographic details of those who have signed up for insurance on the new exchanges.

Health and Human Services Secretary Kathleen Sebelius confirmed that enrollment had surged to 8 million by April 19, after the original deadline was extended to accommodate those trying to sign up on the by March 31 but encountered technical difficulties.

Nearly 2.2 million, or 28 percent, of the total number of enrollees are young adults ages 18 to 34, Sebelius said, noting that the number grows to 2.7 million, or 34 percent, when counting everyone up to age 34.

The percentage of "young invincibles" who have enrolled is key to the health care law’s success because they are healthier and don’t require as much medical care as older Americans. Last summer, the administration set a goal of having 40 percent of enrollees coming from the 18 to 34 age group. If the proportion of young and old enrollees is too lopsided toward older Americans than insurance companies might hike premiums, which could further deter young adults from signing up.

HHS also announced that 62.9 percent of those signing up who agreed to reveal their ethnicity reported they are white, while 16.7 percent said they were African-American, 10.7 percent said they were Latino and 7.9 percent identified as Asian.

Another 1.3 percent said they considered themselves multi-racial, while 0.3 percent said they were Native American Indian or Alaska Natives and 0.1 percent identified as Native Hawaiian or Pacific Islander. Thirty-one percent of enrollees did not report their race or ethnicity or chose “other.”

In addition, Sebelius said more than 4.8 million individuals enrolled in Medicaid and the Children's Health Insurance Program, or CHIP, through the end of March 2014, compared to enrollment before the health exchanges opened last October.

“More than 8 million Americans signed up through the marketplace, exceeding expectations and demonstrating brisk demand for quality, affordable coverage,” she said.

“Together we are ensuring that health coverage is more accessible than ever before, which is important for families, for businesses and for the nation’s health and wellbeing.”

The Obama administration has yet to say how many of the 8 million who signed up on the exchanges have actually paid their first-month insurance premium to officially enroll.

An HHS official, speaking to reporters Thursday, said she doesn’t expect to have those figures until later this year.

The GOP-controlled House Energy and Commerce Committee released a report this week that says just two-thirds of Obamacare enrollees have paid for their health plans by mid-April.

White House spokesman Jay Carney earlier Thursday accused Republicans of distorting partial data.

Another HHS official said he believed the demographics of those who signed up by the first deadline provide enough balance to keep the premiums for insurance plans offered through the new government exchanges stable in every state.

When asked about the net impact of the law – how many of the 8 million who signed up on the exchanges are newly insured versus those who had their plans canceled because of Obamacare — the same official said various public and private estimates of that number are varying so wildly "it’s hard to trust any of these numbers."

This story was first published at 02:21 p.m. and has been updated.