Turning 65 is a milestone most people look forward to. Medicare, on the other hand, is something most people dread figuring out.

The enrollment rules aren't complicated once you understand them, but the penalties for getting the timing wrong are permanent. A missed deadline doesn't just delay your coverage. It adds a surcharge to your monthly premium for as long as you have Medicare, which for most people means the rest of their lives.

This guide lays out exactly what you need to do, when to do it, and what happens if you don't, organized as a clear timeline so nothing falls through the cracks.

First: Understand What You're Enrolling In

Medicare has four parts, and they work differently. Before the timeline makes sense, it helps to know what each one does.

Part A covers inpatient hospital care, skilled nursing facility care, hospice, and some home health services. Most people receive Part A premium-free if they or their spouse worked and paid Medicare taxes for at least 10 years (40 quarters). For those who don't qualify for premium-free Part A, the 2026 premium is up to $518 per month.

Part B covers doctor visits, outpatient care, preventive services, and medical equipment. It comes with a monthly premium — $202.90 in 2026 for most people. This is the part with the most consequential enrollment deadline.

Part C (Medicare Advantage) is an alternative to Original Medicare offered by private insurers. It combines Parts A and B, and usually Part D, into a single plan. You must have both A and B to enroll. These plans typically have lower premiums but use provider networks.

Part D is standalone prescription drug coverage, added to Original Medicare. It's optional but carries a permanent late enrollment penalty if you delay without creditable drug coverage elsewhere.

Beyond these four parts, there's also Medigap (Medicare Supplement), a private policy that fills cost gaps in Original Medicare, covering deductibles, copayments, and coinsurance that Parts A and B don't pay. Medigap works only with Original Medicare, not with Medicare Advantage.

For a deeper look at how the parts work together, see how Medicare works after retirement.

Your Medicare Enrollment Timeline

3 Months Before Your 65th Birthday: Your IEP Opens

Your Initial Enrollment Period begins exactly three months before the month you turn 65. This is the first and best time to enroll.

Why enroll now and not later?

Timing determines when your coverage starts. If you enroll in the three months before your birthday month, your Part B coverage begins on the first day of your birthday month. If you wait until your birthday month or after, coverage is delayed, starting the following month or later. For most people retiring at 65, the goal is seamless coverage from day one. Enrolling in month one or two of your IEP achieves that.

Example: If your 65th birthday is August 15, your IEP runs from May 1 through November 30. Enroll in May, June, or July and your coverage starts August 1.

What to do during this window:

• Decide whether to take Original Medicare (Parts A + B) or Medicare Advantage (Part C)
• If taking Original Medicare, research Medigap plans while you still have guaranteed issue rights (more on this below)
• Enroll online at SSA.gov or call Social Security at 1-800-772-1213

If you're already collecting Social Security at least four months before turning 65, Medicare will enroll you automatically in Parts A and B. You'll receive your red, white, and blue Medicare card in the mail before your birthday. Even so, you'll still need to actively choose a Part D plan or Medicare Advantage plan.

Your 65th Birthday Month: The Middle of Your IEP

Your IEP includes your birthday month. If you enroll during this month, coverage starts the following month — not on your birthday. This is why enrolling in the three months before is preferable if you want no gap in coverage.

Note for birthdays on the first of the month: If you were born on the first day of any month, Medicare treats your birthday as the last day of the prior month. Your IEP starts four months before the calendar month you turn 65 — giving you an extra month of lead time. This also means your Part A coverage can begin a month earlier.

3 Months After Your Birthday: Your IEP Closes

Your Initial Enrollment Period ends three months after the month you turn 65. If you enroll during these final three months, coverage is delayed further — starting one to three months after you sign up. After the IEP closes, you lose the ability to enroll penalty-free unless you qualify for a Special Enrollment Period.

If You're Still Working at 65: The Special Enrollment Period (SEP)

If you're covered by an employer health plan through your own or your spouse's current employment at 65, you can delay Medicare without penalty. This is the most common situation where skipping the IEP makes sense.

Once that employer coverage ends, whether because you retire, your employer drops coverage, or your spouse leaves their job, your Special Enrollment Period begins. You have 8 months from the end of employment or the end of employer coverage (whichever comes first) to enroll in Part B without penalty.

For Part D specifically, the window is tighter: 63 days from the end of creditable drug coverage to enroll in a Part D plan without penalty.

Critical note: COBRA coverage does not count as employer-sponsored group health coverage for Medicare purposes. If you retire and take COBRA, your 8-month Part B window and 63-day Part D window still begin ticking. Waiting for COBRA to run out before enrolling in Medicare is a common, and costly, mistake.

If you're approaching 65 and still working, talk to your employer's HR or benefits administrator now to understand how your current coverage coordinates with Medicare. For more on this, see will I be automatically enrolled in Medicare at 65.

If You Miss the IEP Without a Valid SEP: The General Enrollment Period

If you miss your Initial Enrollment Period and don't qualify for a Special Enrollment Period, you must wait for the General Enrollment Period (GEP), which runs January 1 through March 31 each year. Coverage from a GEP enrollment begins July 1. This means you could face a gap of months without coverage — and you will owe late enrollment penalties.

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The Late Enrollment Penalties: Why Timing Matters So Much

These penalties are not one-time fees. They're permanent monthly surcharges added to your premium for as long as you have Medicare.

Part B Late Enrollment Penalty

For every full 12-month period you were eligible for Part B but didn't enroll (and weren't covered by qualifying employer insurance), your premium increases by 10%. The penalty compounds: two years late means 20% higher premiums, five years late means 50% higher — permanently. At the 2026 standard Part B premium of $202.90 per month, a two-year delay adds roughly $40/month to your bill for the rest of your life.

Part D Late Enrollment Penalty

The Part D penalty accrues at 1% of the national base beneficiary premium for every month you were without creditable drug coverage. In 2026, the national base beneficiary premium is $38.99. Going 18 months without Part D or other creditable coverage adds approximately $7/month — permanently, and the penalty increases as the base premium rises each year.

The penalty kicks in if you go 63 or more consecutive days without creditable drug coverage after your IEP ends. Keep documentation of any employer or union drug coverage you had, you'll need it to appeal if there's a dispute.

The Medigap Window: Don't Miss It

If you choose Original Medicare (Parts A and B) rather than Medicare Advantage, you'll likely want a Medigap policy to cover the out-of-pocket costs Medicare leaves unpaid, deductibles, copayments, and coinsurance.

Your Medigap Open Enrollment Period begins on the first day of the month you are both 65 and enrolled in Part B. It lasts 6 months. During this window, insurers must sell you any Medigap plan they offer in your state, regardless of your health history. They cannot charge you more or deny you coverage because of a pre-existing condition.

After this window closes, insurers in most states can use medical underwriting. They can reject your application, exclude coverage for certain conditions, or charge significantly higher premiums. In some states they can deny you outright.

This is one of the most consequential and time-sensitive decisions in the Medicare enrollment process. If you switch from Medicare Advantage to Original Medicare years later and want to add Medigap, you'll likely face underwriting — and there are no guarantees.

The three most popular Medigap plans are Plan G, Plan N, and Plan F (Plan F is no longer available to new enrollees after January 1, 2020, but those who enrolled before that date can keep it). Plan G is now the most comprehensive plan for new enrollees, covering nearly all out-of-pocket costs except the Part B deductible ($257 in 2025).

Original Medicare vs. Medicare Advantage: The Core Decision

Before you can complete enrollment, you need to make this foundational choice. Neither option is universally better — it depends on your health, finances, and lifestyle.

Original Medicare (Parts A + B + Medigap + Part D)

• See any doctor or specialist in the U.S. who accepts Medicare — no networks, no referrals required
• Costs are predictable if you have a strong Medigap plan
• Higher monthly premiums overall (Part B + Medigap + Part D)
• No annual out-of-pocket maximum from Original Medicare alone, but Medigap fills this gap
• Better suited for frequent travelers, people with chronic conditions, or those who want maximum provider choice

Medicare Advantage (Part C)

• Administered by private insurers replaces Original Medicare as your primary coverage
• Often lower monthly premiums; about two-thirds of plans have no additional premium beyond Part B
• Most plans include Part D drug coverage
• Uses provider networks (HMO or PPO); seeing out-of-network providers may cost more or be uncovered
• Requires prior authorization for many procedures
• Better suited for people in good health who want lower up-front costs and don't mind network restrictions

One important switch consideration: if you enroll in Medicare Advantage and later want to return to Original Medicare with a Medigap policy, you may face medical underwriting. You have a trial period — if you disenroll from Medicare Advantage within the first 12 months and return to Original Medicare, you can get a Medigap policy without underwriting. After that window, the rules change.

For a full comparison, see how Medicare vs. Medicaid differs and how each fits into your retirement plan.

Annual Enrollment Periods: After You're Already Enrolled

Once you're on Medicare, there are two windows each year when you can make coverage changes:

Annual Election Period (AEP): October 15 – December 7. Any Medicare beneficiary can switch between Original Medicare and Medicare Advantage, change Medicare Advantage plans, or add/change/drop Part D coverage. Changes take effect January 1.

Medicare Advantage Open Enrollment Period (OEP): January 1 – March 31. If you're already in a Medicare Advantage plan, you can switch to a different Advantage plan or return to Original Medicare. Limited to one change per year.

These windows don't replace your IEP, they apply to people already enrolled who want to make changes.

IRMAA: The High-Earner Medicare Surcharge

One thing many new Medicare enrollees discover unexpectedly: Medicare premiums are income-based for higher earners. If your income exceeds certain thresholds, you pay an Income-Related Monthly Adjustment Amount (IRMAA) on top of standard premiums.

For 2026, IRMAA surcharges begin at income above $109,000 for individuals and $218,000 for married couples filing jointly. At the highest income tier, Part B premiums can reach $689.90/month per person.

The critical planning detail: IRMAA is based on your income from two years prior. Your 2026 premiums are based on your 2024 tax return. This two-year lag creates situations where retirees, especially those who had a high-income year just before retiring, pay elevated premiums in their first years on Medicare even though their current income is much lower.

If your income has dropped significantly due to retirement, you can appeal your IRMAA using Form SSA-44, documenting the life-changing event. The SSA may use a more recent year's income to recalculate your premium.

For more on how income affects your Medicare costs, see how income affects monthly Medicare premiums.

Planning note: Large Roth conversions, the sale of a home, or a required minimum distribution in the years just before Medicare eligibility can inadvertently push your income over an IRMAA threshold. Since even $1 over a bracket threshold triggers the full surcharge for that tier, income planning in the years approaching 65 is worth reviewing with a financial advisor.

Your Pre-65 Checklist: What to Do and When

12 months before turning 65:

• Decide when you plan to retire and whether employer coverage will continue
• Review the coordination between your employer plan and Medicare; talk to your HR department
• If you have an HSA, stop contributing six months before you plan to enroll in any part of Medicare (Part A enrollment creates retroactive coverage that can create HSA contribution issues)
• Begin researching Medigap plans and Medicare Advantage options in your area

6 months before turning 65:

• Confirm your Social Security status: are you receiving benefits? If yes, expect automatic Medicare enrollment
• If not receiving Social Security, mark your IEP open date on your calendar (3 months before your birthday month)
• Get written confirmation from your employer that your current drug coverage is "creditable" — you'll need this documentation

3 months before turning 65 (IEP Opens):

• Enroll in Part A and Part B at SSA.gov or by calling 1-800-772-1213 (if not auto-enrolled)
• Select a Part D drug plan if staying on Original Medicare
• Apply for a Medigap policy (your guaranteed-issue window opens when Part B begins)
• If choosing Medicare Advantage, compare plans using the Medicare Plan Finder at Medicare.gov

Month of your 65th birthday:

• Confirm coverage is active; review your Medicare card when it arrives
• Verify your chosen plan includes your doctors, specialists, and prescriptions

Within the first 6 months of Part B enrollment:

• Complete your Medigap application if you haven't already — this window closes 6 months from Part B effective date

Frequently Asked Questions

Do I have to enroll in Medicare at 65 if I don't want it?

You're not required to take Medicare, but delaying Part B without qualifying employer coverage will trigger permanent late enrollment penalties. If you're still working and covered by an employer plan (your own or a spouse's), you can delay Part B without penalty and use your Special Enrollment Period when coverage ends.

What if I'm turning 65 but still working and covered by my employer?

You can delay Part B enrollment without penalty as long as your employer coverage remains active. When you stop working or lose coverage, you have 8 months to enroll in Part B and 63 days to enroll in Part D before penalties apply. COBRA does not count as qualifying coverage for this purpose.

Can I have both Medicare and employer insurance?

Yes. If you or your spouse is still working and covered by an employer with 20 or more employees, your employer plan is primary and Medicare is secondary. For smaller employers, the rules may differ. Talk to your HR department to understand how your specific plans coordinate.

What's the difference between Medicare Advantage and Medigap?

Medicare Advantage replaces Original Medicare with a private plan. Medigap supplements Original Medicare by covering out-of-pocket costs. You cannot have both simultaneously. For more, see how Social Security and Medicare work together.

What if I already have retiree health insurance from a former employer?

Retiree coverage typically becomes secondary to Medicare once you turn 65. Enroll in Medicare at 65 regardless — failing to do so may result in your former employer's plan refusing to pay costs Medicare would have covered.

Can I appeal an IRMAA surcharge?

Yes. If your income has declined since the tax year being used to calculate your IRMAA, due to retirement, divorce, death of a spouse, or other life-changing events, you can file Form SSA-44 to request a recalculation using more recent income data.

What happens if I miss Medigap open enrollment?

After your 6-month Medigap open enrollment window closes, insurers in most states can require medical underwriting — and may deny coverage, charge higher premiums, or exclude pre-existing conditions. Some states have expanded protections. Once missed, this window cannot be reopened.

Final Thoughts

The biggest mistake people make with Medicare isn't choosing the wrong plan — it's missing a deadline they didn't know existed.

Your IEP opens three months before your 65th birthday. Your Medigap window opens when Part B begins and closes six months later. Your Part D window is 63 days from when creditable coverage ends. Each of these deadlines carries a different consequence for missing it, and most of those consequences follow you for life.

The good news: all of this is manageable with the right timeline and a little advance preparation. Enrolling in the three months before your birthday month, choosing your coverage type during the Medigap window, and understanding how your current income will affect future premiums are the moves that matter most.

A Retirable advisor can help you think through how Medicare enrollment fits into your broader retirement income plan — including how to time Roth conversions, manage income around IRMAA thresholds, and coordinate Social Security and Medicare to minimize your lifetime costs.

The ideal Alabama retirement starts with a plan.

Let's build your ideal retirement.

Talk with a planning professional at Retirable →