Enrollments in health insurance plans through state and federal exchanges are rising rapidly, especially among young adults, making it likely that the Affordable Care Act will achieve a large and stabilizing mix of enrollees by the end of the open enrollment period on March 31. Some 2.2 million people signed up in the first three months, from Oct. 1 through Dec. 28.

The Department of Health and Human Services reported on Monday that nearly a quarter of them were between the ages of 18 and 34, a group that many experts consider critical to the success of the exchanges because, on average, they are healthier and cheaper to insure than older Americans. One-third were 55 to 64 years old; enrollment in all age groups has been surging now that start-up glitches in the exchanges have largely been fixed.

Some commentators think 40 percent of the enrollees in the health exchange plans need to be in the younger group to keep premiums from spiking. That is a misconception. An analysis last month by the Kaiser Family Foundation estimated that even if that group was roughly 25 percent of the enrollment, it would keep premium increases relatively modest (perhaps 2.4 percent in 2015).

Exactly how the mix of age groups will affect premiums is hard to gauge, because it depends on what insurance companies expected. If enrollees in a plan are sicker than an insurer expected, that might lead the company to raise premiums for 2015. But competition among insurers to attract customers could also keep premiums down.