The three questions that decide everything

1. How big is your employer? 20+ employees changes the rules vs. fewer than 20.
2. Is your employer plan "creditable" for Medicare purposes? (HR can confirm.)
3. Are you still actively working, or are you on COBRA / retiree coverage / severance?

Employer with 20+ employees: usually delay Part B

If you're employed at a company with 20 or more employees AND you (or your spouse) have current employer health coverage, your employer plan is the PRIMARY payer. Medicare would be secondary. This means:

• You can usually delay Part B without penalty. Your Special Enrollment Period (SEP) starts when employment ends and runs for 8 months — plenty of time to enroll then.
• You should still enroll in Part A (which is free for most people based on work history). It can coordinate with employer coverage to lower your costs at hospitals.
• HSA caveat: if you're contributing to a Health Savings Account (HSA), enrolling in ANY part of Medicare (including Part A) makes you ineligible to keep contributing. Stop HSA contributions before enrolling, or delay Part A enrollment until you stop HSA contributions.
• Get the Notice of Creditable Coverage from your HR every year. This is your documentation that proves you had qualifying coverage during the delay — necessary if Medicare ever audits why you delayed enrollment.

Employer with fewer than 20 employees: enroll in Part B at 65

If your employer has fewer than 20 employees, Medicare becomes the PRIMARY payer at 65 — your employer plan becomes secondary. If you DON'T enroll in Part B, your employer plan may pay only what it would have paid as secondary, leaving you with major gaps and potentially huge bills.

Even if you keep employer coverage, enroll in Part B at 65 for this scenario.

The COBRA trap

COBRA is NOT considered "current employer coverage" for Medicare purposes. Once you go on COBRA:

• Your Special Enrollment Period clock starts (8 months from when active employment ended, NOT when COBRA ends)
• If you delay Part B enrollment beyond the 8-month SEP because you're on COBRA, you'll be hit with the Part B late enrollment penalty (10% per year missed, permanent)
• COBRA + Medicare both as your only coverage = redundant + expensive. Most people drop COBRA at 65.

Practical advice: if you retire and go on COBRA at or after 65, enroll in Part B immediately and use Medicare as your primary coverage. Don't try to ride COBRA past 65 — the penalty exposure is too high.

Retiree health coverage

Some employers continue health coverage in retirement (federal government FEHB, some large private employers). Most retiree plans become SECONDARY to Medicare at 65. Like the under-20 employee situation, you usually want to enroll in Part B at 65 even if you're keeping retiree coverage.

Part D while working

Same logic. If your employer plan has "creditable" prescription drug coverage (HR provides annual notice), you can delay Part D without penalty. When you eventually enroll, you have 63 days from when you lose creditable coverage to sign up without penalty.

If your employer plan's drug coverage is NOT creditable, enroll in Part D at 65 alongside Part B (or sooner if you're on a low-employer-coverage situation).

HSA + Medicare = stop contributing

Worth repeating because this catches lots of high-income professionals: enrolling in ANY part of Medicare (including Part A) makes you ineligible to contribute to an HSA. You can still USE existing HSA funds, but you can't add new money.

If you're 6 months from age 65 and want to maximize HSA contributions, time it carefully. Note: when you eventually enroll in Part A, it's retroactive up to 6 months — meaning if you enrolled in HSA in those 6 months, those contributions become disallowed and may need to be removed (with potential tax penalty). This is one of the most common HSA-Medicare planning mistakes.

What to do at retirement

If you're going to retire AT 65: enroll in Part B during your IEP, no SEP needed.

If you're going to retire AFTER 65 with employer coverage: when you give notice, immediately:

1. Get your SEP enrollment paperwork ready (Form CMS-40B + Form CMS-L564 from HR)
2. Submit Part B enrollment within 8 months of your last day of active employment
3. Within 63 days, enroll in Part D OR a Medicare Advantage plan with drug coverage
4. If Medigap is your path, enroll in Medigap as well — your initial 6-month Medigap Open Enrollment Period kicks off when you have BOTH Part B AND age 65+, so timing depends on when you actually enroll in Part B

Special situations

• Self-employed past 65: if you're self-employed, you're "working" but you don't have employer coverage. You should generally enroll in Medicare at 65.
• Spouse covered under your employer: if your spouse is younger than 65 and on YOUR employer plan, your retirement transition affects them too. They may need ACA marketplace coverage to bridge to their own Medicare age.
• VA + employer coverage: VA benefits don't give you a SEP or replace Part B. If you're a veteran working past 65, the rules above still apply to you.

Related: Turning 65 checklist · Late enrollment penalty · Medicare 101