Medicare Enrollment Periods Explained, and the Part B Late Enrollment Penalty

Medicare has several enrollment windows. The main ones are the Initial Enrollment Period around your 65th birthday, Special Enrollment Periods that are triggered by life events such as losing employer coverage, the General Enrollment Period, the Annual Enrollment Period each fall for Medicare Advantage and Part D, and a one-time six-month Medigap open enrollment window that starts when you are both 65 or older and enrolled in Part B. Missing the right window can create a permanent Part B or Part D late enrollment penalty that you pay for as long as you have that coverage.

Last updated: June 2026.

Initial Enrollment Period (IEP)

Your Initial Enrollment Period is a seven-month window around your 65th birthday. It starts three months before the month you turn 65, includes your birthday month, and ends three months after.

• This is when most people first sign up for Part A and Part B.
• You can also enroll in a Medicare Advantage plan or a stand-alone Part D plan during this window.
• Enrolling in the first three months avoids a gap between your 65th birthday and the start of your coverage.

You can apply online at SSA.gov or by calling Social Security at 1-800-772-1213.

Special Enrollment Periods (SEPs)

Special Enrollment Periods let you enroll in or change Medicare coverage outside the normal windows when a qualifying life event happens. Common SEP triggers include:

• Losing employer or union group health coverage while you or your spouse are still actively working.
• Moving out of your plan's service area.
• Qualifying for Medicaid or the Extra Help program for Part D.
• Your Medicare Advantage or Part D plan leaving the Medicare program or your area.

SEPs are important because they generally let you enroll without triggering a late enrollment penalty. The most common one is the eight-month Part B SEP that opens after employer group coverage ends for someone still working past 65. See our guide on Medicare and working past 65 for the full rules.

General Enrollment Period (GEP)

The General Enrollment Period runs January 1 through March 31 each year. It is the fallback for people who missed their Initial Enrollment Period and do not qualify for a Special Enrollment Period. Coverage usually starts the month after you enroll, and a Part B late enrollment penalty may apply if you went without creditable coverage.

Annual Enrollment Period (AEP)

The Annual Enrollment Period runs each fall, October 15 through December 7. During AEP you can:

• Switch from Original Medicare to a Medicare Advantage plan, or back.
• Switch from one Medicare Advantage plan to another.
• Join, change, or drop a stand-alone Part D plan.

Changes you make during AEP take effect January 1 of the following year. AEP is not the window to first apply for a Medigap plan with full guaranteed-issue rights; Medigap has its own one-time window described below.

Medicare Advantage Open Enrollment Period

Each year, January 1 through March 31, people already enrolled in a Medicare Advantage plan get one chance to switch to a different Medicare Advantage plan or to drop Advantage and return to Original Medicare (with the option to add a Part D plan). This window is only for people already on Medicare Advantage on January 1.

The One-Time Medigap Open Enrollment Window

The Medigap open enrollment window is a one-time six-month window that starts the month you are both 65 or older and enrolled in Part B. During this window:

• Medigap carriers in your state must offer you a Medigap policy.
• They cannot use your health history to decline you or charge a higher rate.
• They cannot impose pre-existing condition waiting periods if you have had prior creditable coverage.

This is the easiest moment to lock in a Medigap plan. Outside this window, except in specific guaranteed-issue situations, Medigap carriers in most states can ask health questions and your answers can affect whether you are accepted and at what rate. For a deeper look at the most common standardized Medigap plans, see Medigap Plan G vs Plan N.

The Part B Late Enrollment Penalty

The Part B late enrollment penalty is one of the most expensive mistakes in Medicare because it is permanent. The federally standardized rule is:

• For each full 12-month period you could have had Part B but did not, 10% is added to the standard Part B premium.
• You generally pay this higher premium for as long as you have Part B.

We do not state the current-year Part B premium dollar amount here because it changes every year. For the current Part B premium, see Medicare.gov. The penalty itself is a percentage rule, so it compounds on whatever the current-year premium happens to be.

People most at risk of the Part B penalty are those who delay Part B without having other creditable coverage, such as active employer group coverage from an employer of the right size. If you are still working past 65, do not assume any coverage qualifies. See Medicare and working past 65 for the specific rules and the eight-month Part B Special Enrollment Period that opens when employer coverage ends.

The Part D Late Enrollment Penalty

The Part D late enrollment penalty is also permanent. The federally standardized rule is:

• For each full month you went without Part D or other creditable prescription drug coverage after you were first eligible, 1% of the national base beneficiary premium is added to your Part D premium.
• You generally pay this higher premium for as long as you have Part D.

We do not state the current national base beneficiary premium dollar amount here because it changes every year. For the current figure, see Medicare.gov. Even if you take no medications today, enrolling in a low-cost Part D plan or maintaining other creditable drug coverage protects you from this penalty later.

How This Connects to Income (IRMAA)

Late enrollment penalties are separate from IRMAA. IRMAA is the income-related extra amount that higher-income beneficiaries pay on top of their Part B and Part D premiums, regardless of how they enrolled. See our IRMAA pillar for how IRMAA is calculated, how the two-year lookback works, and how to appeal.

Frequently Asked Questions

What is the Initial Enrollment Period for Medicare?

The Initial Enrollment Period is a seven-month window around your 65th birthday. It starts three months before the month you turn 65, includes that month, and ends three months after. This is when most people first sign up for Part A and Part B.

What is the Medicare Annual Enrollment Period?

The Annual Enrollment Period runs each fall, October 15 through December 7. You can join, switch, or drop a Medicare Advantage plan or a Part D prescription drug plan, and changes take effect January 1.

What is the Medigap open enrollment window?

The Medigap open enrollment window is a one-time six-month window that starts the month you are both 65 or older and enrolled in Part B. During this window Medigap carriers in your state cannot use your health to decline you or raise your rate.

How does the Part B late enrollment penalty work?

The Part B late enrollment penalty is 10% added to the standard Part B premium for each full 12-month period you could have had Part B but did not, and it is paid for as long as you have Part B. We do not list current-year dollar amounts because the Part B premium changes every year.

How does the Part D late enrollment penalty work?

The Part D late enrollment penalty is 1% of the national base beneficiary premium for each full month you went without Part D or other creditable drug coverage after you were first eligible. It is added to your Part D premium for as long as you have Part D.

What is a Special Enrollment Period?

A Special Enrollment Period is triggered by a qualifying life event, such as losing employer coverage or moving out of a plan's service area. SEPs let you enroll in or change Medicare coverage outside the normal windows without the late enrollment penalty.

What is the General Enrollment Period?

The General Enrollment Period runs January 1 through March 31 each year. It is the fallback for people who missed their Initial Enrollment Period and do not qualify for a Special Enrollment Period. Coverage starts the month after you enroll, and the Part B late enrollment penalty may apply.

What if I am still working at 65 with employer coverage?

If you are still actively working and have group health coverage from an employer with 20 or more employees, you can usually delay Part B without a late enrollment penalty, and you get an eight-month Special Enrollment Period to sign up for Part B after that coverage ends. See Medicare and working past 65.

References

• Medicare.gov: When does Medicare coverage start
• Medicare.gov: Medicare costs at a glance
• SSA.gov: Medicare enrollment

We are not connected with or endorsed by the federal government or the Medicare program.

We do not offer every plan available in your area. Any information we provide is limited to those plans we do offer in your area. Please contact Medicare.gov or 1-800-MEDICARE to get information on all of your options.

Not sure which Medicare window applies to you right now? I am a licensed independent agent based in Omaha. I can map out your specific timeline so you do not get hit with a permanent late enrollment penalty. Free consultation, no pressure.

Book a free Medicare consultation with Nick Depke or call (402) 680-6171.

Written by Nick Depke, licensed independent insurance agent (NPN 19158595), Depke Insurance Agency, 17310 Wright St Ste 100, Omaha, NE 68130.