Many people work past 65 or have coverage through a spouse’s job. If that sounds familiar, the key questions are when to enroll in Medicare, which coverage pays first, and whether delaying Part B could create a penalty or coverage gap.

The answer depends on facts such as current employment, employer size, the type of coverage, disability or End-Stage Renal Disease status, and whether you have retiree or COBRA coverage. This article is general education, not individualized advice.

Primary and secondary payer basics

When two health plans cover the same person, one generally pays first and the other may consider the remaining covered costs. The plan that pays first is the primary payer. The other is the secondary payer. Secondary coverage does not necessarily pay every remaining bill.

The Medicare Secondary Payer guidance explains that employer size and the reason for Medicare eligibility can affect which coverage pays first.

If you are 65 or older and actively working

For people who are eligible for Medicare because of age, a common rule is:

If the employer has 20 or more employees, the employer group health plan generally pays first and Medicare pays second.
If the employer has fewer than 20 employees, Medicare generally pays first and the employer plan pays second.

“20 employees” is not a universal answer for every situation. Ask how the employer counts employees and how its plan coordinates with Medicare. The employer’s benefits administrator and Medicare can confirm the rule for your circumstances.

When Medicare is primary, delaying Part A or Part B can leave the employer plan refusing or reducing payment for services Medicare should have paid first. That is why people at smaller employers should be especially careful before declining Medicare.

Can you delay Part B without a penalty?

If you or your spouse are still working and you have group health coverage based on that current employment, you may qualify for an 8-month Special Enrollment Period to enroll in Part B after the employment or coverage ends. Medicare’s working-past-65 enrollment guidance says the timing matters even if you choose COBRA or another form of coverage after the job ends.

Before delaying Part B, ask the employer plan administrator to confirm in writing:

Whether the coverage is based on current employment.
Whether the employer plan pays primary or secondary.
Whether you must enroll in Part A, Part B, or both.
Whether enrolling in Part A affects a Health Savings Account (HSA).
When coverage ends and what forms Social Security will need later.

If you contribute to an HSA, get tax guidance before enrolling. Medicare enrollment generally makes you ineligible to contribute to an HSA, and premium-free Part A can begin retroactively in some situations. That can create excess-contribution tax issues. Ask Medicare, your employer benefits administrator, and a tax professional when contributions should stop.

Why employer size matters

The employer plan may have different payment responsibilities based on whether the employer has 20 or more employees. A plan that pays secondary may expect Medicare to process the claim first. If you decline Medicare when it should be primary, you could face unpaid or delayed claims.

This rule is separate from deciding whether employer coverage is affordable or whether a Medicare plan seems attractive. Compare the payment order first, then compare premiums, networks, prescriptions, and cost-sharing.

COBRA and Medicare

COBRA is continuation coverage after certain employment or coverage-ending events. It is not the same as active employer coverage. If you already have Medicare and later elect COBRA, Medicare generally pays first and COBRA pays second for services covered by both, subject to the plans’ rules. The coordination rules can be different for people whose Medicare eligibility is based on ESRD.

COBRA coverage can also affect enrollment timing. Medicare’s official guidance warns that the 8-month Part B Special Enrollment Period is tied to the end of current employment or job-based coverage, not simply the end of COBRA. Get advice before relying on COBRA to postpone Medicare enrollment.

Retiree coverage is different too

Retiree coverage is not active-employment coverage. It may require you to enroll in Medicare Part A and Part B, and it may pay after Medicare. Read the retiree plan materials and ask what happens when you become Medicare-eligible.

Do not drop retiree coverage until you understand whether you can get it back. Medicare’s retiree insurance guidance can help you prepare questions.

Part D and employer drug coverage

Going without Medicare Part D or other creditable prescription drug coverage for 63 or more consecutive days after the applicable Initial Enrollment Period ends may result in a Part D late-enrollment penalty later. See Medicare’s Part D penalty rules.

A practical transition checklist

Ask the employer benefits office for the plan’s Medicare coordination rules.
Confirm whether the employer has 20 or more employees under the applicable rule.
Identify the date current employment and job-based coverage will end.
Compare Medicare’s Part A, Part B, and prescription coverage timing with the employer plan.
Ask about HSA contributions before enrolling in any part of Medicare.
If using COBRA or retiree coverage, treat it as a separate decision, not automatically as a Part B delay protection.
Keep plan notices, proof of coverage, and Social Security enrollment records.
Recheck the rules before the transition. Medicare does not publish 2027 plan premiums or benefits in this article, so use current official information when those details become available.

Before you act

Confirm payer order, enrollment timing, HSA effects, and coverage end dates with Medicare, Social Security, your employer benefits administrator, and a tax professional when appropriate. Do not delay enrollment based on this article alone.

How Medicare Works with Employer Health Insurance