People 55 to 64 -- the range just below the age at which people qualify for Medicare -- represented the largest group, at 31 percent, down from 33 percent in the months from October through December.

The open enrollment period continues until March 31, and White House officials predict a surge of applications just before the deadline.

People who go without insurance after that may be subject to tax penalties, although the Internal Revenue Service has indicated that it prefers public education over aggressive enforcement in the first year of the “individual mandate.”

After reviewing the new report, Caroline F. Pearson, a vice president of Avalere Health, a research and consulting company, said: "Enrollment in the exchanges appears to be on track to reach roughly six million by the end of March. It is also important that the age mix of enrollees improved in January, relative to the first three months of open enrollment. That supports my hypothesis that the older, sicker enrollees will sign up first, and the younger, healthier people will enroll later.”

After reviewing the new report, Caroline F. Pearson, a vice president of Avalere Health, a research and consulting company, said: “Enrollment in the exchanges appears to be on track to reach roughly six million by the end of March. It is also important that the age mix of enrollees improved in January, relative to the first three months of open enrollment. That supports my hypothesis that the older, sicker enrollees will sign up first, and the younger, healthier people will enroll later.”

The administration and its allies are planning a big push to sign up more people, in the hope that total enrollment through the exchanges could reach the administration’s original goal of seven million by the end of March.