Health and Human Services Secretary Kathleen Sebelius said "for those keeping score at home" a total of about 9 million people have received health insurance coverage under aspects of the ACA so far. That total includes sign-ups in private Obamacare plans through the exchanges, another 3.9 million people additional people signed up in government-run Medicaid or CHIP programs, and 3 million people under age 26 who are by law covered by their parents' health plans under the ACA.
"We're seeing a very strong response to the marketplace," Sebelius said, referring to HealthCare.gov and the 15 other Obamacare exchanges run by states and the District of Columbia.
Sara Collins, an analyst at the Commonwealth Fund, told CNBC.com, "I think it's an excellent report."
"I think what's particularly striking is the large number who selected a plan in December," Collins said. "If you were to see that level of enrollment in the next three months, you could well exceed that 7 million [enrolled] that CBO estimated."
Collins was referring to a Congressional Budget Office estimate that 7 million would enroll in Obamacare plans by the March 31. That estimate had been made before it was known that HealthCare.gov and several state-run exchanges had had serious technical problems that would take months to fix after they launched Oct. 1.
Collins said she also expected the proportion of young adults to increase by March 31, citing a recent Commonwealth Fund survey which found that 41 percent of the visitors to the Obamacare exchanges were that age group.
(Read more: Health spending becomes a smaller share of GDP)
Jost, the health law professor, said older, less healthy people who either had insurance before the exchanges launched, or who didn't, were more likely to have been motivated to sign up for coverage through the exchanges by Jan. 1 than young adults.
"There's good reason to believe that young people will wait as long as possible to sign up," Jost said, calling the sign-up rate among young adults "pretty remarkable" so far.
He added, "If they don't get to 40 percent, it wouldn't the end of the world."
Jost cited a recent Kaiser Family Foundation that found that under a worst-case scenario in which young adults made up just 25 percent of enrollees, insurers would likely increase premium rates next year by just 2 percent or so.
"I don't think it's going to be a disaster if they don't get higher than this," Jost said.
—By CNBC's Dan Mangan. Follow him on Twitter