Even if insurance pools contain only 25 percent young adults rather than the hoped-for 40 percent, medical claims and other costs would exceed premium revenue by only about 2.4 percent, . That’s far below the kind of loss that would lead to an unsustainable spiral of huge premium increases and fewer and fewer subscribers, they say. (Kaiser Health News is an editorially independent program of the foundation.)

“Premiums are not as sensitive to the mix of enrollment as fears about a ‘death spiral’ suggest, particularly with respect to age,” wrote Â鶹ŮÓÅ’s Larry Levitt and colleagues. More important, they add, is that insurers recruit a balanced mix of the healthy and the sick of all ages.

Many deem enrolling “young invincibles” to subsidize older, sicker members as essential to the policies sold through healthcare.gov and other portals created by the Affordable Care Act. That goal has gained attention as some states have disclosed the ages of early enrollees. In California, for example, in October and November were aged 18 to 34.

But under ACA guidelines, insurers charge young adults as little as one-third what they charge a 64-year-old. (Younger adults are still paying more than their expected average medical costs, however.) That limits subsidies from the young and means that the importance of enrolling them might be overestimated, the Â鶹ŮÓÅ analysts suggested. In other words, younger folks who stay uninsured deprive insurers of less revenue than an equal number of uninsured older adults would.

“My numbers aren’t immensely different from the Kaiser numbers,” said Seth Chandler, a law professor and insurance specialist at the University of Houston. “Kaiser is a little more upbeat in the way they’re thinking about it than I am.”

that lower levels of young-adult enrollment could generate losses as high as 10 percent, a figure he said “is not wildly different” from the one produced by the Â鶹ŮÓÅ model and one that wouldn’t necessarily doom Obamacare.

The key, both Â鶹ŮÓÅ and Chandler say, is to recruit enough healthy members no matter what their age.

“It is important to attract the ‘young invincibles,’ but maybe with a greater focus on the ‘invincible’ part,” the Â鶹ŮÓÅ study said.

Many worry that consumers with expensive illness will be the first to sign up and healthy folks, who don’t think they need immediate coverage, will be inclined to stay away — especially given the sign-up portals’ computer problems.

The Â鶹ŮÓÅ analysts expect young-adult enrollment to increase with the approach of the March 31 signup deadline to avoid penalties for lacking coverage. Even if some carriers experience sicker-than-average members and high medical claims, the ACA includes risk-sharing mechanisms to reduce carriers’ exposure and potential need to raise premiums, many have noted.

Chandler, whose blog is entitled  is less confident it will all work out.

President Obama’s for existing policyholders will tend to keep healthy people out of plans falling under the health law, which could drive up average costs, he said. He thinks risk-sharing mechanisms might need  to do their job of stabilizing the market.

In any event, what’s most important is not enrollees’ birthdays but how sick they are. Nobody will know that for months.

“People have focused on age because that is the only data we have,” Chandler said. “But the real issue is the health status of the people we enroll.”

Â鶹ŮÓÅ Health News is a national newsroom that produces in-depth journalism about health issues and is one of the core operating programs at Â鶹ŮÓÅ—an independent source of health policy research, polling, and journalism. Learn more about .

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