Why Timing Matters
Medicare has specific windows for enrollment and changes. Miss a window and you could face permanent penalties, limited plan options, or months without coverage. This is one area where the rules are strict and the consequences are real.
Initial Enrollment Period (IEP)
When: 7 months centered on your 65th birthday (3 months before, your birthday month, 3 months after)
What you can do: Enroll in Part A and Part B for the first time. This is also when you should enroll in a Medigap plan (your Medigap Open Enrollment Period starts when you have Part B and are 65+).
If you have employer coverage: You may be able to delay enrollment without penalty. But the rules are specific: the coverage must be from current employment (not COBRA, not retiree coverage). Talk to someone before assuming you can delay.
Annual Enrollment Period (AEP)
When: October 15 through December 7 each year
What you can do:
- Switch from Original Medicare to Medicare Advantage (or vice versa)
- Change your Medicare Advantage plan
- Join, switch, or drop a Part D plan
Changes take effect January 1 of the following year. This is the main window for plan changes.
Medicare Advantage Open Enrollment Period (MA OEP)
When: January 1 through March 31 each year
What you can do:
- Switch from one Medicare Advantage plan to another
- Drop Medicare Advantage and return to Original Medicare (and pick up a Part D plan)
This is not a window for people on Original Medicare to join Medicare Advantage. It’s only for people already in a Medicare Advantage plan.
Special Enrollment Period (SEP)
When: Triggered by qualifying events
Examples:
- You lose employer coverage (you get 8 months)
- You move out of your plan’s service area
- You qualify for Medicaid
- Your plan is terminated or reduces coverage
- You were affected by a natural disaster
SEPs have different rules depending on the triggering event. Don’t assume you have one without verifying.
General Enrollment Period (GEP)
When: January 1 through March 31 each year
What it’s for: Enrolling in Part B if you missed your IEP and don’t qualify for a Special Enrollment Period. Coverage starts July 1.
The catch: You’ll pay a late enrollment penalty of 10% of the standard Part B premium for each full 12-month period you could have had Part B but didn’t. This penalty lasts for as long as you have Part B.
Medigap Open Enrollment Period
When: 6 months starting the month you’re 65 or older and enrolled in Part B
Why it matters: During this window, insurance companies cannot deny you a Medigap policy or charge you more because of health conditions. After this window closes, they can do both (in most states).
This is the most important window that most people don’t know about.
The Bottom Line
Medicare enrollment timing is unforgiving. Late enrollment penalties for Part B and Part D are permanent. Missing your Medigap Open Enrollment Period can mean paying significantly more for coverage, or being denied entirely.
If you’re approaching 65, leaving employer coverage, or helping a family member navigate this, a 15-minute call can prevent expensive mistakes.
For official enrollment resources and phone numbers, see our Resources page. You can also call 1-800-MEDICARE (1-800-633-4227) with enrollment questions.