A Reuters report (link sent to me by Chris Conover; thanks Chris) reports that in four states where they have reports about 20% of new enrollees are 18-34 years old (ranging from 19% in KY to 27% in MD). The report indicates that "The Obama administration is aiming to enroll about 2.7 million 18-to-35 year olds in the exchanges by the end of March, out of 7 million total, or about 38 percent."

The age distribution seems actually better than I might have expected at this early stage.  Many reports suggest that young folks (and healthy) are often the last to enroll.  So no doubt the percentage who are younger will rise over time, especially at the end of the enrollment period (March).  As one analyst says in the article, drawing conclusions from this early data is like "trying to call an election when you only have 10 percent of the returns."