Insurers say the early buyers of health coverage on the nation's troubled new websites are older than expected so far, raising early concerns about the economics of the insurance marketplaces.

If the trend continues, an older, more expensive set of customers could drive up prices for everyone, the insurers say, by forcing them to spread their costs around. "We need a broad range of people to make this work, and we're not seeing that right now," said Heather Thiltgen of Medical Mutual of Ohio, the state's largest insurer by individual customers. "We're seeing the population skewing older."

The early numbers, described to The Wall Street Journal by insurance executives, agents, state officials and actuaries, are still small—partly a consequence of the continuing technical problems plaguing the federally run exchanges, experts say. HealthCare.gov, the federally run marketplace serving 36 states, is suffering serious technical problems that have prevented many people from signing up.

But the numbers demonstrate a real-world fallout from the digital snafus: Less-healthy customers are more likely to persevere through technical obstacles to gain coverage, insurers say. Younger, healthier customers who feel less need for insurance—but whose widespread participation is important to the financial success of the system—could be quicker to give up.

The average enrollee age at Priority Health, a Michigan insurer, has ticked up to age 51 for newcomers, from about 41 years old for plans offered for the current year, said Joan Budden, chief marketing officer. Arise Health Plan, Wisconsin's largest nonprofit insurer, said more than half its 150 signees are over 50, a higher proportion than expected, while declining to be specific on its target age.