But getting “young invincibles,” as insurers sometimes call them, to sign up for insurance is an uphill climb. Even with the public campaigns, only about one in four 19- to 29-year-olds is even aware of the exchanges where they might buy affordable insurance, and the ignorance is especially acute among the uninsured, according to a survey this year by the Commonwealth Fund, a nonprofit research group.

“There’s very low awareness among young adults,” said Sara R. Collins, an economist with the Commonwealth Fund. “It’s a concern in states that aren’t actively promoting these exchanges. People might remain unaware,” she said, referring to the 36 states that have opted to let the federal government run their exchanges for them. They include Texas and Mississippi, where public officials are campaigning against the health law.

That lack of awareness makes it all the more important that those who do know about HealthCare.gov — and try to purchase insurance there — are not dissuaded because of the glitches, the analysts said. Older and sicker Americans have a stronger incentive to keep trying to sign up despite the clunky site, they said.

Though economists, insurers and health analysts are concerned about the problems with HealthCare.gov, which the Obama administration has promised to fix by Nov. 30, they said it was too early to tell whether the problems would cause an underenrollment of the young and healthy. Insurers would have a good sense of any problems by next spring, they said.

No statistics are available on how many of them have signed up. States are providing no demographic details on enrollees. And the Obama administration has declined to say how many people have purchased insurance in the 36 states where it runs the exchanges.

Jonathan Gruber, an economist with the Massachusetts Institute of Technology who helped create the Affordable Care Act, said lessons could be drawn from Massachusetts, which in 2006 implemented a similar law to provide near-universal coverage in the state.

Many Massachusetts residents waited until just before the state law’s tax penalty kicked in before signing up for insurance, he said. Just 123 people signed up for subsidized insurance in the first month of enrollment, and only about 2,000 in the second month.