The Enrollment Windows That Set Your Costs for Life

Medicare has five distinct enrollment periods, each with its own rules and consequences. The system rewards planning and punishes assumptions. Below, every window worth memorizing.

Initial Enrollment Period (IEP)

The 7-month window around your 65th birthday: the 3 months before your birthday month, your birthday month, and the 3 months after. Enroll in Part A and Part B during this window unless you have creditable employer coverage. If you enroll in the 3 months before, coverage starts the month you turn 65; enrolling later delays your start.

General Enrollment Period (GEP)

January 1 – March 31 each year. For people who missed their IEP and don't qualify for a Special Enrollment Period. Coverage now begins the month after you enroll (a 2023 improvement over the old July 1 start).

Annual Election Period (AEP)

October 15 – December 7. When most beneficiaries do their annual plan shopping. During AEP you can:

• Switch from Original Medicare to a Medicare Advantage plan or vice versa.
• Switch between Medicare Advantage plans.
• Join, switch or drop a Part D plan.

Changes take effect January 1.

Medicare Advantage Open Enrollment (MA-OEP)

January 1 – March 31. Available only to people already enrolled in Medicare Advantage. You can make one switch — to a different MA plan or back to Original Medicare with a standalone Part D plan. You cannot use this window to switch from Original Medicare into an Advantage plan.

Special Enrollment Periods (SEPs)

Triggered by qualifying life events: losing employer coverage, moving out of a plan's service area, entering or leaving Medicaid, moving into or out of a nursing facility, a plan losing its CMS contract, or a federally declared disaster. Most SEPs give you a 60- to 63-day window. Document the triggering event.

The penalties

• Part B late enrollment penalty: 10% of the standard premium for every full 12-month period you were eligible but not enrolled. Permanent. Added to your Part B premium for life.
• Part D late enrollment penalty: 1% of the national base beneficiary premium ($36.78 in 2025) for every uncovered month, added permanently.
• Part A late enrollment penalty: Applies only to the small share of beneficiaries who must buy Part A (10% surcharge for twice the number of years you were late).

Working past 65

If your employer coverage is creditable — from a group health plan at an employer with 20 or more employees — you can delay Part B without penalty and enroll during an 8-month SEP after employment ends. Get the creditable-coverage confirmation from the plan in writing before you rely on this rule.

When your window opens, take the plan comparison checklist into the Medicare Plan Finder guide.

Frequently asked questions

When can I compare and switch Medicare plans?

The Annual Election Period runs October 15 to December 7, with new coverage starting January 1. If you already have a Medicare Advantage plan, you get a second window from January 1 to March 31 to switch plans or return to Original Medicare.

What is the Initial Enrollment Period?

A seven-month window around your 65th birthday: the three months before your birthday month, the birthday month itself, and the three months after. Enrolling before your birthday month gives you coverage starting the first day of that month.

What happens if I miss a Medicare enrollment deadline?

Part B carries a permanent 10% premium penalty for every full 12 months you were eligible but not enrolled, and Part D adds 1% of the base premium for each uncovered month. Both penalties last as long as you have the coverage.

Can I delay Medicare if I am still working?

Yes, if your employer coverage is creditable and the employer has 20 or more employees. You then get an eight-month Special Enrollment Period after that employment or coverage ends. Get written confirmation of creditable coverage before relying on this.