As a disclaimer, this information is for the first three months of enrollment prior to Jan. 1, but it is directly from the Department of Health and Human Services.
Here is the information from Business Week, with my emphasis in bold, and edits in brackets.
About 30 percent of new enrollees are under 35. White House officials say that’s an acceptable mix, and they expect more young people to come on board closer to the March 31 deadline. “We think that more and more young people are going to sign up as time goes by, based on the experience in Massachusetts,” Gary Cohen, deputy administrator at the Centers for Medicare and Medicaid, said on a conference call with reporters. “We’re actually very pleased with the percentage that we have right now, and we expect that percentage to increase.”
Most of the people who bought coverage [four out of five] on the exchanges this fall got subsidies to help them afford the premiums. That’s in contrast to the first month of the program, when less than one-third of buyers were subsidized. People earning up to four times the poverty rate—as much as $96,000 a year for a family of four—can get help buying coverage.