President Barack Obama sat down with Zach Galifianakis last month with one goal in mind: Convince young people to buy insurance. (AP Photo/Funny Or Die, File)

Some states running their own Obamacare health insurance marketplaces reported a modest bump in youth enrollment in March, suggesting that many young adults waited until the deadline to sign up for coverage as expected.

However, enrollment in most of those states is short of the Obama administration's goal last summer to have young adults between 18 and 34 years old account for 40 percent of marketplace enrollments. Obama administration officials in January said, though, they were already confident enough young people have already enrolled to support their health care law.

Five state-based health insurance marketplaces and the District of Columbia's exchange have so far provided an age breakdown of people who signed up for coverage through March 31, the official end of open enrollment. Compared against numbers reported by the Department of Health and Human Services in January and February, young adults in March signed up at higher rates.

These numbers don’t tell the whole story of youth enrollment, though. HHS hasn’t reported March demographic data for the 36 states where it’s operating the marketplaces. Nine other state-based exchanges also haven’t yet provided an age breakdown for March enrollment.

People who struggled to sign up before March 31 can still enroll before April 15 through, the federal enrollment portal. HHS said it doesn’t have a reliable estimate of how many are still trying to enroll. State-run exchanges have also allowed some deadline leeway.

The numbers also don’t account for young adults who enrolled in coverage outside the exchanges. The online Web broker eHealthInsurance, which sells off-exchange plans nationwide, recently reported that 45 percent of its shoppers since Jan. 1 are between 18 and 34 years old. Insurers must consider exchange customers and non-exchange customers as part of the same risk pool if they’re selling in both markets.

Youth outreach was the Obama administration’s main focus in the final weeks of enrollment because of how important young, healthy enrollees are to supporting the exchanges. It’s why President Barack Obama sat down with Zach Galifianakis on “Between Two Ferns,” and celebrity allies of the White House last month boosted outreach efforts.

Now that the Affordable Care Act bans insurers from charging people more or denying coverage based on health status, healthy people are especially important to help offset the costs of sicker patients in the insurance market. Because sicker patients would have a greater motivation to obtain insurance, the health care law included the requirement for almost everyone to get coverage or pay a fine.

Through the end of February, young adults accounted for 25 percent of total exchange sign-ups nationwide, though their enrollment pace slightly increased in January and February. The administration hasn’t released March enrollment figures yet, and the White House declined to comment for this story.

Insurance market experts said there isn’t one specific target for youth enrollment. The makeup of each state’s insurance market and insurers’ 2014 expectations will matter much more for 2015 rates, they said.

“From a carrier’s standpoint, the more young, healthy lives the better,” said Elizabeth Carpenter, director at Avalere Health. “But I think that each carrier has made their own assessment to what kind of mix they need to be successful.”

Young doesn’t necessarily mean healthy. However, insurers will set 2015 rates without having a lot of information about how much care their new customers are seeking, so age is a guidepost for how healthy the risk pool is.

The ACA includes several programs meant to stabilize premiums if the enrollment mix is less healthy than expected. A Kaiser Family Foundation analysis from December found a youth enrollment rate of 25 percent could increase premiums 1 percent to 2 percent.

Rhode Island, a state with an older population, saw youth enrollment in its exchange increase from 26 percent of total sign-ups in February to 28.5 percent in March.

“It’s a good number – we had a bump,” said Christine Ferguson, the director of Rhode Island’s exchange. “I expect that as we go throughout the course of the year and into next year, there’ll be an even better distribution.”

Correction: This post was updated to reflect that the Obama administration originally set the 40 percent target for young adult enrollment, not the Congressional Budget Office.