If you’re still employed and working at a company or organization with 20 or more employees (or your spouse is), and you’re covered by an employee health plan, you may not need Part B yet. Instead of paying premiums, it could make financial sense to hold off. But it’s important to know that after losing employee coverage — due to retirement, layoffs or any other reason — you have an eight-month “special enrollment period” to sign up for Part B.

If you miss that window, you have to wait for the general enrollment period, which runs from January 1 through March 31 each year. That creates two problems.

First, Medicare will add a permanent 10 percent penalty to your premiums for each year you delayed. Mr. Zeppenfeldt-Cestero has to pay $187.60 per month now because he waited more than three years to sign up. If he had enrolled when he was 65, his monthly premium would be $134.

“Without good information, people make mistakes and they’re costly,” Dr. Neuman said. “In this case, it’s a cost that continues the rest of your life.”

The second problem: the general enrollment period imposes coverage gaps.

The general enrollment period (not to be confused with the Open Enrollment Period currently underway when those already receiving Medicare can change plans) runs from January through March, remember. But coverage doesn’t begin until the following July 1.

If you didn’t sign up for Part B and recognize your error in March, you can be insured in July. If you figure it out in April, however, you can’t enroll until the following January and coverage begins the following July.

“It could be well over a year, depending on when you discover the problem,” said David Lipschutz, senior policy lawyer at the Center for Medicare Advocacy. “Unfortunately, some people discover it when they get sick.”