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.