Medicare Initial Enrollment: The Seven-Month Window You Can't Miss

Medicare enrollment isn't automatic for most people. Most pre-retirees assume the federal government just turns Medicare on the day they turn 65. It does not. There is a seven-month window — three months before your 65th birthday month, the birthday month, and three months after — when you sign yourself up. Miss that window without qualifying employer coverage and you pay a lifetime penalty for being late, every single month, for the rest of your life. The penalty isn't huge in any single month. Compound it across thirty years of retirement and it's a real number. Let me walk through how to not be one of the people who gets caught, right?

The seven-month Initial Enrollment Period

Your Initial Enrollment Period (IEP) is:

• The three calendar months before your 65th birthday month, plus
• Your 65th birthday month, plus
• The three calendar months after your 65th birthday month.

So if you turn 65 in August 2026, your IEP runs from May 1, 2026 through November 30, 2026.

During this window you can enroll in Medicare Part A (hospital coverage), Part B (medical/outpatient coverage), and a Part D prescription drug plan. You can also choose between Original Medicare with a Medigap supplement or Medicare Advantage at this time. Most people who enroll during their IEP face no late-enrollment penalty on any part of Medicare.

One quirk: if you enroll before your birthday month (the first three months of your IEP), your coverage starts the first day of your birthday month. If you enroll during or after your birthday month, coverage start date depends on the month — generally the first day of the month after enrollment, with some lag.

What happens if you're already collecting Social Security at 65

If you started Social Security before age 65 — for example, you claimed at 62 — you are automatically enrolled in Medicare Part A and Part B effective the first day of your 65th birthday month. SSA mails you a Medicare card a few months before your birthday. You don't need to do anything — except actively choose whether to keep Part B or decline it (rare; usually if you have employer coverage), and choose whether to add a Part D plan and either Medigap or Medicare Advantage.

If you're not collecting Social Security yet, the enrollment is on you. Apply through SSA's website (ssa.gov/medicare) or by calling SSA. Most people enroll online; it takes about 15 minutes.

The exception: creditable employer coverage

If you're still working at 65 and have creditable group health coverage from an employer with 20 or more employees, you can defer Part B enrollment without late-enrollment penalty. You stay on the employer plan until it ends, then you have an 8-month Special Enrollment Period to sign up for Part B without penalty.

Two crucial caveats here:

• "20 or more employees" matters. Smaller employers (under 20) typically make Medicare primary at 65 — meaning Medicare pays first and the employer plan pays second. In that situation, you should enroll in both Part A and Part B at 65 even though you have employer coverage. Confirm with your employer's HR.
• "Creditable coverage" is a defined term. Most large-employer group plans qualify, but a few don't. Get the determination in writing.

Most people enroll in Part A at 65 even with employer coverage, since Part A is usually premium-free for people who have 40 quarters of Medicare-taxed work history. Part B is the one with the standard premium ($206.50/month in 2026), so the deferral is mostly about avoiding that monthly cost while you're on the employer plan.

Caution about Part A and HSAs: if you contribute to a Health Savings Account, enrolling in Part A — even premium-free — disqualifies further HSA contributions. We covered this in the working-past-65 article. If you're trying to keep HSA contributions going, you need to delay Part A as well, which means delaying Social Security past 65 (since SS auto-enrolls you in Part A).

If you miss the Initial Enrollment Period

Two paths back in:

• General Enrollment Period: January 1 through March 31 of each year. If you missed your IEP and don't qualify for a Special Enrollment Period, you can sign up during the GEP. Coverage starts the month after enrollment. Late-enrollment penalty applies if you didn't have qualifying coverage during the gap.
• Special Enrollment Period: If you had qualifying employer coverage, you have 8 months from the end of that coverage to enroll without penalty.

The late-enrollment penalties

The Part B penalty is the one that hurts most. For someone who delays Part B by three years without qualifying coverage, the 30% surcharge means an extra $62/month for the rest of their life. Over a 25-year retirement, that's roughly $18,600 in extra premiums for a three-year gap.

The four enrollment-window scenarios

Most pre-retirees fall into one of these four buckets:

• Retiring before or at 65, no employer coverage carrying forward: Enroll during your seven-month IEP. This is the simplest path.
• Working past 65 with creditable employer coverage from a 20+ employer: Defer Part B. Enroll in Part A only (unless you have HSA reasons to delay). When employer coverage ends, use your 8-month Special Enrollment Period.
• Working past 65 with employer coverage from a small employer (under 20): Enroll in both Part A and Part B at 65. Medicare is primary; employer plan is secondary.
• Already collecting Social Security at 65: Auto-enrolled in Part A and B. Check your mailbox for the Medicare card. Choose between Original Medicare + Medigap vs. Medicare Advantage. Add a Part D plan unless your employer plan covers prescriptions.

The "wait, I have COBRA" trap

One more trap. If your employer coverage ends and you elect COBRA continuation, COBRA does not count as creditable coverage for Medicare's purposes. The 8-month Special Enrollment Period started the day your active employment ended — not the day COBRA ends. People who go on COBRA for the full 18 months and then try to enroll in Medicare often find they've blown through their SEP and face a late-enrollment penalty.

If you're between jobs at 65 or considering COBRA, enroll in Medicare during your IEP or SEP first. Don't rely on COBRA as a bridge.

What this looks like in practice

The seven-month Initial Enrollment Period is one of those bureaucratic windows where the rules are clear and the penalties are real, but the warning isn't as loud as it should be. SSA sends you a Medicare card if you're already on Social Security. SSA does not proactively flag your IEP if you're not on Social Security. You're expected to know.

If you're approaching 65, the right move is to start the conversation 12 to 18 months early. Confirm whether you have creditable employer coverage. Confirm whether your spouse's coverage applies. Confirm whether your situation involves any of the small-employer or HSA wrinkles. Make the enrollment decision intentionally — not by deadline default. Sleep at night, knowing you didn't step on the federal-government version of a rake.

The bottom line

Your Medicare Initial Enrollment Period is the seven months centered on your 65th birthday month. Enroll then, or qualify for an exception (creditable employer coverage from a 20+ employer plus the 8-month SEP afterward), or face a lifetime late-enrollment penalty. Auto-enrollment happens only if you're already collecting Social Security at 65. COBRA does not count as creditable coverage. The penalty for getting it wrong follows you forever. Start the conversation early.