Ever since the Affordable Care Act’s insurance marketplaces opened for business in 2014, the Obama administration has worked hard to get Americans to sign up. Yet officials now are telling some older people that they might have too much insurance and should cancel their marketplace policies.

Each month, the Centers for Medicare and Medicaid Services is sending emails to about 15,000 people with subsidized marketplace coverage. The message arrives a few weeks before their 65th birthday, which is when most become eligible for Medicare.