After a slow and rocky start, the state and federal health insurance exchanges seem to finally be taking off. By the end of December, enrollment surpassed the 3 million mark for state and federal marketplaces combined.

But young adults — who many claim are key to the success of the Affordable Care Act — haven't been as eager as their older counterparts to enroll.

Adults between the ages of 18 and 34 make up 40 percent of the potential market for exchange plans. But a report published Jan.13 by the Department of Health and Human Services (HHS) shows they only accounted for a quarter of total enrollees.

Experts say there are many factors that have kept young adults away from the exchanges, including an unintended consequence from another provision of the law. But there's widespread agreement that better efforts are needed to encourage them to sign up.

The individuals most likely to go the exchange are those who need the coverage the most — those with chronic conditions, said Health Policy Analyst Jordanna Davis, senior associate at Sachs Policy Group in New York. This is also the population that will cost insurance companies the most.

Plans need healthier people — generally, the young — who contribute more than they use in benefits to balance things out financially, said Davis, who also consults on, a website launched by UPMC to help explain healthcare reform and the marketplace to patients.

Because young people are generally healthy, they are more likely than older adults to believe they are invulnerable to major illness or medical conditions, said Julie Chase, president and CEO of Chase Communications, a national public relations and health policy communications firm. This is driving their healthcare decision-making.

Chase conducted a nationwide survey of more than 1,000 adults in December 2013. It found that 62 percent of younger adults aged 18 to 34 believe they are less at risk for a major illness or medical condition than the average American. That percentage is even larger for the youngest adults (18-24) at 66 percent. By comparison, only 50 percent of Americans aged 45 to 54 and 52 percent over the age of 55 feel they are less at risk.

Their lack of interest in the insurance exchanges goes deeper than just their attitudes, said David Oscar, an insurance broker in New Jersey.

"It's about disposable income," he said. "We, for decades, have had a problem with getting young people signed up for coverage."

The ACA addressed the difficulty these young, lower-wage earners have had with affording coverage by allowing them to stay on their parents' policies until age 26, even if they are otherwise financially independent. This provision of the law, which went into effect in 2010, could be having an unintended consequence on the enrollment numbers for the exchange plans.

Now that these young adults have been given the ability to stay on their parents' insurance, it's not something the government is going to take away from them, Oscar said. At the same time, the kids aren't going to voluntarily give up a good policy to hit the individual market, he said. According to HHS, more than 3 million young adults gained coverage through their parents' insurance because of the ACA.

"It certainly wasn't a flaw to give all of those kids really important coverage for those three years," Davis said.

Davis would not go as far as to call it a flaw that they are still allowed on their parents policies now that the exchanges are open. But she does see how it makes sense from an economic standpoint to include the 26 and younger population in the risk pool that is the worst.

Chase said the government, advocates, healthcare professionals and the plans themselves have to do a better job of communicating the importance of coverage to the young adult population. It's going to take more than a "folksy" message from President Barack Obama urging parents to tell their kids to buy insurance, Chase said, referring to a remark Obama made during his Jan. 28 State of the Union address.

Offering young adults smarter ways to shop for insurance that take into account their needs and budgets will help move things in the right direction, said John Park, chief strategy officer at Alegeus Technologies, a Waltham, Mass.-based technology provider for the benefit services industry. Alegeus recently entered the exchange market as the technology vendor for a private insurance exchange in Florida.

Park said he expects to see better cooperation between the private and public insurance marketplaces in the future. He hopes that better advocates will also emerge who will help young adults navigate the market and educate them on the nuances, and importance, of buying insurance.