Most people enroll in Medicare during their Initial Enrollment Period — a 7-month window starting 3 months before the month they turn 65 and ending 3 months after. Missing this window can result in permanent late-enrollment penalties for Part B and Part D, so the timing matters.
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Medicare has multiple enrollment windows. Choosing the right one — and understanding the consequences of missing it — is one of the most important Medicare timing decisions you will make. The Social Security Administration and Medicare.gov's enrollment overview are the authoritative sources; what follows is an educational summary.
Your Initial Enrollment Period is 7 months long: it begins 3 months before the month you turn 65, includes the month you turn 65, and ends 3 months after. For someone turning 65 in September, the IEP runs June 1 through December 31. This is the most straightforward window. If you are already receiving Social Security or Railroad Retirement Board benefits before 65, you are generally enrolled in Medicare Parts A and B automatically; you receive your Medicare card by mail before your 65th birthday.
If you (or your spouse) are still actively working at 65 and enrolled in employer-sponsored health insurance through that active employment, you may delay Part B without penalty. You have a Special Enrollment Period: you can enroll in Part B at any time while covered by the employer plan, or during the 8-month window that starts the month after employment or coverage ends — whichever comes first. Retiree coverage, COBRA, and coverage from a spouse's former employer do NOT count as active employer coverage for this purpose. The Medicare.gov SEP guide clarifies what qualifies.
If you did not enroll during your IEP and do not qualify for a Special Enrollment Period, you must wait for the General Enrollment Period (GEP): January 1 through March 31 each year, with coverage starting July 1. You will also be assessed a Part B late-enrollment penalty: 10% added to your Part B premium for each full 12-month period you were eligible but did not enroll. This penalty is permanent — it stays with you for as long as you have Part B.
The Part D late-enrollment penalty works similarly: for each full month you go without creditable prescription drug coverage after your IEP ends, 1% is added to your Part D premium. The penalty is calculated against the national base beneficiary premium (a figure CMS sets annually), and it is also permanent. Having creditable drug coverage from another source — such as an employer plan — pauses the penalty clock. Medicare.gov's Part D penalty page has the current formula.
Once enrolled in Medicare, you can switch between Medicare Advantage plans or between Original Medicare and Medicare Advantage during the Annual Election Period (AEP): October 15 through December 7 each year, with coverage changes effective January 1. There is also a Medicare Advantage Open Enrollment Period: January 1 through March 31, allowing those already in a Medicare Advantage plan to switch to a different Medicare Advantage plan or return to Original Medicare once.