Despite a late surge that led to 8 million Americans signing up for coverage through President Obama's health care law, just 28 percent of those signing up were from the crucial younger demographic, according to new data from the Department of Health and Human Services. The number is below the original HHS target.

The health care law defied early expectations after a botched rollout and ended up seeing a surge of 3.8 million Americans selecting an insurance plan through one of the law's exchanges in March, along with an extended enrollment period that was measured through April 19.

HHS did not release data on the number of individuals who completed the enrollment process by paying their premiums, noting that it “does not yet have comprehensive and accurate data about effectuated enrollment (that is, the number of individuals who have effectuated their enrollment and gained coverage through payment of the first month’s premium). However, some issuers have made public statements indicating that 80 percent to 90 percent of the people who have selected a Marketplace plan have made premium payments.”

Though there was a surge of 1.2 million signups coming from the ages 18-to-34 demographic (representing 31 percent of enrollees, an increase from earlier months), the final number of 2.2 million signups, out of 8 million total, was just 28 percent.

Before the launch of the open enrollment process in October, White House officials were saying 2.7 million of an expected 7 million enrollees (or nearly 40 percent) should be from the younger demographic in order to offset the cost of insuring older and sicker enrollees.