Overview: Enrollment timing is one of the most financially consequential aspects of Medicare, and the rules are specific enough to appear repeatedly on licensing exams.

The Three Key Enrollment Windows

Initial Enrollment Period (IEP): The first and most important window. It spans seven months: the three months before the month you turn 65, the month you turn 65, and the three months after. If you enroll in the first three months of the IEP, coverage begins on the first day of your birthday month. If you enroll during or after your birthday month, coverage is delayed by one to three months. Missing the IEP entirely without a qualifying exemption triggers permanent penalties.

Annual Enrollment Period (AEP): Runs every year from October 15 through December 7, with coverage changes taking effect January 1. During the AEP, any Medicare beneficiary can switch between Original Medicare and Medicare Advantage, change Advantage plans, or join, drop, or switch a Part D plan.

Special Enrollment Period (SEP): A qualifying life event outside the standard windows that allows enrollment without penalty. The most common SEP is losing employer-sponsored group health insurance — you have eight months to sign up for Parts A and B after employer coverage ends. For Part D, the window is shorter: two months after losing creditable drug coverage.

Delayed Enrollment Penalties

Part B penalty: If you fail to enroll during your IEP and do not qualify for a Special Enrollment Period, your monthly Part B premium increases by 10% for every full 12-month period you were eligible but not enrolled. This surcharge lasts for life.

Example: Standard 2026 Part B premium = $202.90. If you delayed enrollment by two full years, your penalty = 20% × $202.90 = $40.58/month. Your total monthly premium = $243.48, paid permanently.

Part D penalty: Already described above — 1% per uncovered month, permanent.

FAQ

If I'm still working at 65 and have employer coverage, do I face a penalty?
Generally no, as long as the employer has 20 or more employees. You qualify for a Special Enrollment Period once employment or employer coverage ends, allowing you to enroll in Parts B and D without penalties. However, COBRA coverage and retiree health coverage do not count as "active employment" coverage for this purpose — those do not exempt you from the penalty rules. Always clarify the source of coverage before advising a client to delay enrollment.

What is "creditable coverage" for Part D?
Creditable coverage is prescription drug insurance that is, on average, at least as good as standard Medicare Part D coverage. Employers, unions, and certain other entities that provide drug coverage must notify beneficiaries each year whether their coverage is creditable. If you receive this notice and your coverage is creditable, you can delay Part D enrollment without penalty.