An analysis last month by the Kaiser Family Foundation suggested that fears that under-enrollment by young adults will hurt insurers' bottom lines are greatly overstated. That analysis calculated that if 18-to-34-year-olds enrolled at a rate 25 percent lower than other individuals relative to the potential market, then the added costs to insurers from health-care expenses and overhead would be just 1.1 percent higher than premium revenue.
In what Kaiser called the "likely worst-case scenario," in which that age group enrolled at a 50 percent lower rate, overall costs in plans would be about 2.4 percent higher than premium revenue.
In either scenario, insurers would still likely book a profit—albeit lower than the profit they had priced their plans for.
"Even in the worse case, insurers would still be expected to earn profits, and would then likely raise premiums in 2015 to make up the shortfall," Kaiser wrote. "However, a 1 to 2 percent premium increase would be well below the level that would trigger a 'death spiral,' which would occur if insurers needed to increase premiums substantially, in turn further discouraging young and healthy people from enrolling."
Insurers are also protected, analysts and government officials noted, by provisions in Obamacare including a reinsurance program that compensates plans sold on the exchanges for costs that exceed their revenue.
More changes ahead
If federal officials did release demographic data now, it would not be the last word in what the profile of total enrollment will be at the close of open enrollment on March 31, Jost said. There is "no question" that the demographic profile will change by then, he said.
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"I think we're going to get a lot more subsidy-eligible people, a lot more younger people, I think the age profile will certainly change," Jost said.
HHS spokeswoman Peters also suggested that would be the case, saying that Obamacare "is making health insurance more affordable for young adults."
"With nearly half of single, marketplace eligible uninsured young adults able to get coverage at $50 or less per month, the health-care law is delivering the quality, affordable coverage people are looking for," Peters said.
—By CNBC's Dan Mangan. Follow him on Twitter