Although more young people signed up for health insurance, for example, the number remained below the level that some analysts believe is optimal for keeping premiums low in the insurance marketplaces. The administration said Thursday that 28 percent of those who bought policies were between the ages of 18 and 34, but some analysts said the optimum level would be 40 percent.

“In an ideal world, you’d want to get as close to that as possible,” said Larry Levitt, a senior vice president at the Kaiser Family Foundation. “But what is important is what the insurance companies expected, and this is what they expected.”

The administration did not release two other crucial statistics that would help determine the success of the law: the number of people among the eight million who bought insurance for the first time and the number who paid their initial premiums.

The number of those who were previously uninsured is important, since many people could simply have been moved from plans that were canceled by the law. Administration officials have promised to release that information when they have it, but they have said it is not data that is collected by the government.

Administration officials have said previously that they could not tell how many people have paid premiums because those payments are between individuals and their insurance companies. In February, however, industry experts and insurance companies said that one in five people who signed up failed to pay their premiums on time and therefore did not receive coverage when it began in January.