Staying Fit
You should probably enroll during the seven-month window known as your initial enrollment period. It begins three months before the month of your 65th birthday, includes that month, and ends three months after the birthday month. For example, if you turn 65 on July 4, 2023, the enrollment window is open from April 1 to October 30.
If you miss this deadline, you may have to pay higher premiums for Medicare Part B, which covers doctor bills, outpatient fees and other medical expenses. However, some people who are still working at 65 and have group health insurance through their employer (or a spouse's employer) may be able to delay enrollment without this penalty.

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Keep in mind
If you are already receiving Social Security benefits when you turn 65, the Social Security Administration will automatically enroll you in parts A and B of Medicare. (Part A covers hospitalization.) You can opt out of Part B, for example if you have health insurance through an employer or other source.
More on retirement
Can I get Medicare if I’m not getting Social Security yet?
Can I enroll in Medicare if I claim Social Security retirement benefits at age 62?
How do Social Security and Medicare work together?