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In this series, “Social Security Secrets,” experts Larry Kotlikoff and Phil Moeller explain the most important things you need to know about this valuable retirement benefit. Kotlikoff and Moeller are co-authors of the New York Timesbest-seller Get What’s Yours: The Secrets to Maxing Out Your Social Security.

It used to be commonplace to enroll in Social Security and Medicare at age 65. Times have changed. More and more Americans are working later into their 60s. Unfortunately, they are missing very specific enrollment periods for Medicare. In most cases, missing these enrollment periods can lead to penalties that can follow a Medicare recipient for a lifetime.

An important detail to note is that if you or your spouse is covered through a current employer's insurance—but not through COBRA—you can put off filing for Medicare. Another important point: If you need to get Medicare at age 65, you have a seven-month window to file for it—the calendar month in which you turn 65, plus the three months preceding that month and the three months after it. This is known as the "initial enrollment period."