Medicare and Employer Coverage at 65: The 20-Employee Rule Explained

The Question Everyone Turning 65 Gets Wrong

You're approaching your 65th birthday, you have perfectly good health insurance through your employer, and you're planning to keep working. Do you need to sign up for Medicare? The answer depends on a single fact about your employer that most people never think to check: how many employees work there.

Get this wrong and you could end up with a permanent premium surcharge that follows you for the rest of your life. Get it right and you'll either save on premiums by delaying enrollment or protect yourself from a devastating coverage gap.

How the 20-Employee Rule Works

Federal Medicare Secondary Payer rules draw a hard line at 20 employees:

Your employer has 20 or more employees. Your employer's group health plan pays first (primary payer), and Medicare pays second. You can delay Part B enrollment without any penalty. Your employer plan covers you exactly as it always has. When you eventually retire and lose that coverage, you'll have an 8-month Special Enrollment Period to sign up for Part B penalty-free.

Your employer has fewer than 20 employees. Medicare is the primary payer from the month you turn 65. Your employer plan drops to secondary status, meaning it only covers what's left after Medicare pays. If you haven't enrolled in Part B, there's no primary payer for outpatient services. Your employer plan — now secondary — may pay little or nothing on those claims.

The employee count includes all workers at the company, not just those on the health plan. Part-time employees count. The threshold looks at whether the employer had 20 or more employees during 20 or more calendar weeks in the current or previous calendar year.

What "Active Employment" Actually Means

The safe-to-delay rule only applies when your coverage is based on current, active employment — either your own or your spouse's. This distinction matters enormously because two very common types of coverage don't qualify:

COBRA continuation coverage starts after active employment ends. Even if you're paying the same premiums to the same insurer, COBRA doesn't count as active employment coverage. Your 8-month Special Enrollment Period starts when active employment ended, not when COBRA expires.

Retiree health benefits are offered by some employers as a retirement perk. They're not based on active employment either. Medicare is primary over retiree health plans, so you need to enroll in Part B during the 8-month Special Enrollment Period after active employment ends.

What to Do If You're at a Large Employer (20+)

If your employer has 20 or more employees, here's your playbook:

1. Enroll in premium-free Part A. It's free if you or your spouse have 40+ quarters of Medicare-taxed work, and it provides hospital coverage that coordinates with your employer plan. There's no penalty for late Part A enrollment when you qualify for free Part A.

2. Delay Part B. You're saving $202.90/month (the 2026 standard premium) by staying on your employer plan without Part B. There's no financial advantage to having both.

3. Watch your HSA. If you contribute to a Health Savings Account, enrolling in Part A makes you ineligible for further HSA contributions. You'll need to decide whether free Part A or continued HSA contributions is the better deal. Stop HSA contributions at least 6 months before enrolling in Part A to avoid the retroactive eligibility problem.

4. When you retire, file immediately. Submit Form CMS-40B (Part B enrollment application) and Form CMS-L564 (employer coverage verification) to Social Security within your 8-month SEP window.

What to Do If You're at a Small Employer (Under 20)

If your employer has fewer than 20 employees, the math changes completely:

1. Enroll in Part A and Part B during your Initial Enrollment Period. Your IEP is the 7-month window centered on your 65th birthday — 3 months before, the birthday month, and 3 months after.

2. Keep your employer plan as secondary coverage. It fills gaps that Medicare doesn't cover, like copays and deductibles. But Medicare must be paying first, or you'll have claims denied.

3. Tell your employer. Your employer's benefits administrator needs to update their records to reflect that Medicare is now primary. This ensures claims are processed correctly from day one.

4. Don't assume your employer knows the rules. HR departments at small companies frequently give incorrect Medicare guidance. They may tell you that you don't need Medicare because you have "perfectly good insurance." That advice can cost you thousands in denied claims and permanent penalties.

How to Verify Your Employer's Size

Counting employees sounds straightforward, but companies near the threshold create real problems:

• Check the employer's annual 5500 filing (if they have one) for participant counts
• Ask HR directly how many total employees the company has — stress total, not just those on the health plan
• If the company was recently acquired or merged, the post-transaction employee count is what matters
• Staffing agencies and PEOs can complicate the count — check whether the employees are technically employed by the agency or the client company

If your employer hovers around 20 employees — say, 18 one year and 22 the next — consult the Benefits Coordination & Recovery Center at 1-855-798-2627 to confirm your coordination status.

The Medicare Late-Enrollment Penalties and Special Enrollment guide includes a headcount audit worksheet that walks through the exact counting methodology, plus the CMS-L564 packet you'll need when it's time to transition.

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