Medicare Enrollment Periods Explained (IEP, AEP, SEP, GEP).

Half of the calls I take are about timing. Somebody turns 65 next month, or six months from now, or last March and they didn't realize they were supposed to do anything. The Medicare rules have four or five named enrollment windows depending on how you count, and each one has its own job. Miss the right one and you can be paying a penalty every month for the rest of your life — or stuck on a plan that doesn't fit you for an entire year.

The goal of this article is to walk you through every Medicare enrollment window in plain English, tell you which one applies to your situation, and show you what each one actually lets you do.

The windows at a glance

When is my Initial Enrollment Period?

Your IEP is a 7-month window built around the month you turn 65 — the three months before, the month itself, and the three months after. This is the first and most important enrollment window for most people. If you sign up during the three months before your birthday month, your Medicare coverage starts the first day of your birthday month. If you sign up later in the window, your coverage starts later. Miss the window entirely and you can owe a permanent Part B late-enrollment penalty.

There are two big exceptions:

1. If you're already getting Social Security benefits before you turn 65, you'll be automatically enrolled in Parts A and B. Your Medicare card arrives in the mail about three months before your birthday. You don't have to do anything — but you still want to review whether to keep both parts (and what to add for drugs, doctors, and out-of-pocket protection).
2. If you're still working past 65 with qualifying group coverage, your timeline is different. We cover that situation in Medicare and Working Past 65. The short version: you may be allowed to delay Part B without penalty, but the rules depend on how many people work at the company.

Our shortest piece of advice: start looking eight or nine months before your 65th birthday. That gives you room to compare plans without rushing.

What is the Annual Enrollment Period (AEP)?

The Annual Enrollment Period runs October 15 through December 7 every year. Any change you make during AEP takes effect January 1.

What AEP is for:

• Switching from one Medicare Advantage plan to another.
• Switching from Medicare Advantage back to Original Medicare (and adding a Part D drug plan).
• Switching from Original Medicare to a Medicare Advantage plan.
• Joining, switching, or dropping a stand-alone Part D drug plan.

What AEP is not for: starting Medicare for the first time (that's your IEP), and switching Medicare Supplement (Medigap) plans. Supplements have their own rules, and in Missouri the friendliest rule is the anniversary window — our Missouri Anniversary Rule article covers it in detail.

A small but important point: you can make multiple changes during AEP if you want. The last change you submit before December 7 is the one that takes effect on January 1.

What is the MA-OEP?

The Medicare Advantage Open Enrollment Period runs January 1 through March 31 every year. It's a smaller window with a narrower job.

If you're already in a Medicare Advantage plan on January 1, MA-OEP gives you one switch. You can change to a different Advantage plan, or you can drop the Advantage plan and go back to Original Medicare (and pick up a Part D drug plan at the same time).

What you can't do during MA-OEP:

• Switch from Original Medicare to Medicare Advantage (that's an AEP move, not an MA-OEP move).
• Join or change a Part D plan if you're staying on Original Medicare.
• Switch Medicare Supplements without going through medical underwriting (most of the time — Missouri has a friendlier rule once a year, covered in Article 8).

MA-OEP is what we use when somebody enrolled in an Advantage plan during AEP and within a few weeks realized their primary care doctor wasn't in network, or the drug list dropped a medication they take. It's the safety valve for AEP.

What is a Special Enrollment Period?

Special Enrollment Periods are time-limited windows triggered by qualifying life events. These are the most commonly missed enrollment opportunities, because most folks don't realize they qualify.

The most common SEPs in our office:

• Losing employer or union coverage. You usually get an 8-month window to enroll in Part B, and a 2-month window to join a Medicare Advantage plan or Part D drug plan, after the coverage ends.
• Moving. Moving to a new state — or even to a different county in Missouri — opens a window to change your Advantage or Part D plan, because plan availability is location-based.
• Losing Medicaid eligibility. A specific Medicaid-related SEP applies.
• Your plan leaves the Medicare program (rare) or changes its contract. You get a window to pick a new plan.
• 5-star plan SEP. If a 5-star-rated Advantage or Part D plan is available in your county, you have a once-per-year window (December 8 through November 30) to switch into it.
• Qualifying for Extra Help / LIS (the federal program that helps low-income beneficiaries pay for prescription drugs).

If you think you might qualify for an SEP, call before you make any changes. The penalty for guessing wrong is locking yourself out of the plan you wanted for a full year.

What is the General Enrollment Period?

The General Enrollment Period runs January 1 through March 31 every year. It's for people who missed their Initial Enrollment Period and don't qualify for any Special Enrollment Period.

If you enroll during GEP, your Medicare coverage starts the first of the month after you enroll. You may also owe a Part B late-enrollment penalty based on how long you delayed enrolling. The penalty is permanent.

GEP is the "I missed my IEP and I'm not eligible for an SEP" backstop. We don't want anyone to be in this situation, but it exists.

What if I missed a deadline?

This is the question we get most, and the answer depends on which deadline you missed.

Missing your IEP for Part B

You'll usually owe a Part B late-enrollment penalty of 10% of the standard Part B premium for every full 12-month period you delayed. On the 2026 standard premium of $202.90, each full year you delayed adds 10% — about $20.29 per month — to your Part B bill, and that surcharge is permanent. Two full years late is roughly $40.58 a month; three years about $60.87. The math is unforgiving.

Missing your IEP for Part A

Most people don't pay a Part A premium because they paid Medicare taxes through their working years. If you're one of the people who has to pay for Part A and you delay, the penalty is 10% of the Part A premium, paid for twice the number of years you delayed. (We see this rarely.)

Going without creditable Part D drug coverage

The Part D late-enrollment penalty is 1% of the national base beneficiary premium for every month you went without creditable drug coverage. It gets added to whatever Part D plan you eventually join, for as long as you have Part D.

The good news on penalties: most of them are avoidable. Our Late-Enrollment Penalties article covers penalty math in real numbers, the rules around what counts as "creditable coverage," and the rare cases where equitable relief is available.

A St. Louis example, start to finish

If you decided not to enroll in Part B during your IEP because you have group coverage at work, the timing changes when you eventually retire — that triggers an 8-month SEP for Part B.

Common mistakes around timing

Five we see often:

1. Waiting until the last month of the IEP. Processing time can run into your start date. Give yourself a 3-month cushion.
2. Assuming AEP is when you first enroll. It isn't. AEP is for people already on Medicare.
3. Assuming COBRA counts as creditable coverage for Part B. It doesn't. COBRA is creditable for Part D, but not for Part B.
4. Letting an SEP expire without acting. SEPs are time-limited. The clock starts the day the qualifying event happens (or the day you got the notice, depending on the rule).
5. Switching plans during MA-OEP thinking you can also switch Supplements. MA-OEP only deals with Advantage and Part D — Supplements have their own rules.

What to do next

If your IEP is open or coming up, download our free Turning-65 Medicare Checklist (St. Louis Edition). It walks you through the 6 months before your birthday week by week, so you don't miss anything.

If you think you might qualify for an SEP and you're not sure, call us at (314) 248-6500. We answer every call. There's no fee for a conversation, no pressure, and no sales pitch — that's the rule of the house.

I've been helping St. Louis folks navigate this since 2013. Timing is the easiest mistake to avoid, and the most expensive one to make.