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Medicare Enrollment Periods Explained: What Tucson Seniors Need to Know

One of the most common — and costly — Medicare mistakes is missing an enrollment window. This guide explains every Medicare enrollment period so you can make the right moves at the right time.

Why Enrollment Timing Matters

Unlike most insurance, you can't enroll in Medicare just any time. Medicare has specific enrollment windows, and missing them can result in permanent financial penalties that follow you for the rest of your life. Getting the timing right — especially around your 65th birthday — is one of the most important Medicare decisions you'll make.

1. Initial Enrollment Period (IEP) — Your First and Best Opportunity

The Initial Enrollment Period is the 7-month window surrounding your 65th birthday. It starts 3 months before the month you turn 65, includes your birthday month, and extends 3 months after. This is the most important enrollment window for most people.

During your IEP, you can enroll in Medicare Part A and Part B. When you enroll matters for when coverage starts:

• If you enroll in the 3 months before your birthday month, coverage starts on your birthday month
• If you enroll in your birthday month, coverage starts the following month
• If you enroll in the 3 months after your birthday month, coverage starts 1–3 months after you enroll

Bottom line: Enroll as early as possible during your IEP — ideally 3 months before your 65th birthday — to ensure seamless coverage with no gap.

2. Special Enrollment Period (SEP) — For Those Still Working

If you're still working at 65 and covered by employer health insurance (from a company with 20+ employees), you can delay Medicare enrollment without penalty. This is one of the most misunderstood areas of Medicare, and mistakes here are extremely costly.

When your employer coverage ends (either because you retire or lose coverage), you have an 8-month Special Enrollment Period to sign up for Medicare Part A and Part B without facing a late penalty.

Important caveats:

• The 8-month SEP begins when employment ends OR when the employer group coverage ends — whichever comes first
• COBRA coverage does not count as creditable employer coverage for SEP purposes — do not delay enrollment based on COBRA
• Marketplace (ACA) plans do not count as creditable coverage for Medicare purposes — if you're on a Marketplace plan at 65, enroll in Medicare on time
• If your employer has fewer than 20 employees, Medicare is primary and you should enroll during your IEP regardless

3. General Enrollment Period (GEP) — If You Missed Your Window

If you missed your IEP and don't qualify for an SEP, the General Enrollment Period runs from January 1 through March 31 each year. Coverage begins July 1. This is the "catch-up" window, but it comes with consequences: late enrollment penalties for both Part B and, if applicable, Part D.

4. The Part B Late Enrollment Penalty — A Lifelong Cost

If you don't sign up for Part B when first eligible and don't qualify for an SEP, you'll pay a 10% permanent premium increase for every 12-month period you were eligible but didn't enroll. For example, if you delayed Part B by 2 years, your Part B premium is permanently increased by 20% — for the rest of your life.

In 2025, the standard Part B premium is $185/month. A 20% penalty adds $37/month, or $444/year — every year, forever.

5. Annual Enrollment Period (AEP) — Switching Plans Each Fall

Every year from October 15 through December 7, Medicare beneficiaries can make changes to their Medicare Advantage and Part D plans. Changes take effect January 1. During AEP you can:

• Switch from one Medicare Advantage plan to another
• Switch from Medicare Advantage back to Original Medicare
• Join, switch, or drop a Part D prescription drug plan

This is the most important annual review period. Plan benefits, premiums, and formularies can change each year — so what was the best plan for you last year may not be the best plan this year. We recommend an annual review every fall.

6. Medicare Advantage Open Enrollment Period (OEP)

From January 1 through March 31 each year, if you're already enrolled in a Medicare Advantage plan, you can make one plan switch: either to a different MA plan or back to Original Medicare (and enroll in a standalone Part D plan). This is a one-time switch opportunity each calendar year.

7. Medigap Open Enrollment — A One-Time Window

If you want to purchase a Medicare Supplement (Medigap) policy, your best chance is during your Medigap Open Enrollment Period: a 6-month window starting the first month you're 65 or older AND enrolled in Part B. During this window, no insurer can deny you coverage or charge more based on your health. After this window, you may be subject to medical underwriting — which can result in higher premiums or denial of coverage.

This is a one-time window. Unlike Medicare's annual enrollment periods, the Medigap Open Enrollment Period does not repeat every year.

Quick Reference Summary

Need Help Navigating Enrollment?

Medicare enrollment is one of the most consequential financial decisions you'll make. Getting it wrong can cost you thousands in penalties and coverage gaps. Our local Tucson Medicare advisors are here to walk you through every step — completely free, with no obligation.

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