Medicare Enrollment Deadlines: Miss These and Pay Penalties Forever

Initial Enrollment Period (IEP)

Your Initial Enrollment Period is a 7-month window around your 65th birthday:

• 3 months before the month you turn 65
• The month you turn 65
• 3 months after the month you turn 65

If you enroll during the first 3 months, coverage starts the first day of your birthday month. If you enroll in month 4 or later, your coverage start is delayed 1–3 months. Enroll as early as possible to avoid gaps.

Special Enrollment Period (SEP)

If you're covered by employer-sponsored insurance (your own or a spouse's) when you turn 65, you can delay Medicare without penalty. When that coverage ends, you have an 8-month SEP to enroll in Part A and B.

Important: COBRA and retiree coverage do NOT count as "employer coverage" for SEP purposes. If you lose employer coverage and rely on COBRA, your SEP clock started when employment ended, not when COBRA ends.

General Enrollment Period (GEP)

If you miss your IEP and don't qualify for an SEP, you must wait for the General Enrollment Period: January 1 – March 31 each year. Coverage begins July 1 of that year — meaning you could have up to 6 months without coverage.

Worse, you'll likely pay a permanent Part B premium penalty: 10% added for each full 12-month period you were eligible but not enrolled. Miss 2 years? Your premium goes up 20% — forever.

Part D Late Enrollment Penalty

The Part D penalty is calculated as 1% of the national base beneficiary premium for every month you were without creditable drug coverage. In 2024, the base premium is about $34.70/month, so each month of delay adds roughly $0.35/month to your premium — permanently.

Example: Miss Part D for 24 months = 24% penalty ≈ $8.33/month extra, every year for life.

Employer Coverage Exception Rules

You're exempt from penalties if you had coverage through an employer with 20 or more employees during the gap period. Smaller employer plans may not qualify — check with your HR department and get written confirmation that your employer coverage is "creditable."

For Part D, any drug coverage that's "at least as good as Medicare" counts as creditable — including employer plans, VA coverage, and TRICARE. Your insurer must send you an annual notice of creditable coverage.

Medicare Advantage and Part D Open Enrollment

Each year, Medicare Advantage and Part D have an Annual Enrollment Period (AEP): October 15 – December 7. Changes made during AEP take effect January 1. There's also an Open Enrollment Period (OEP) from January 1 – March 31 specifically to switch Medicare Advantage plans or return to Original Medicare.

Key Dates Summary