Turning 65 while you’re still working changes the standard advice about signing up for Medicare, because whether you need Part B right away depends mainly on the size of your employer’s health plan. Get the size wrong and you can end up with a permanent premium penalty or a gap between your job coverage and Medicare, even though you never stopped working.
Does Your Employer’s Size Decide When You Sign Up for Medicare?
Yes, in most cases. Medicare Supplement (Medigap) plans come later in this decision, but the first question is always about your current job-based coverage: how many employees does the company have? If the answer is 20 or more, Medicare generally treats your job coverage as primary and lets you delay Part B without a penalty. If it’s fewer than 20, Medicare typically becomes primary the month you turn 65, whether or not you’re still working. Your HR administrator can confirm which side of that line you fall on.
20 or More Employees: You Can Usually Delay Part B
When your employer (or your spouse’s employer) has 20 or more employees, your group health plan counts as coverage based on current employment, and Medicare lets you put off Part B without a late penalty for as long as that coverage lasts. Once the job ends or the group coverage ends, whichever happens first, you get an 8-month Special Enrollment Period to sign up for Part B. Miss that window and you’re back to waiting for the General Enrollment Period (January 1 through March 31) with a penalty attached. Source: Medicare.gov.
REMEBER: COBRA and retiree coverage don’t count as active employment for delaying Medicare and they don’t extend your 8-month window. If you plan to take COBRA after you retire, sign up for Part B the moment you end your work rather than waiting for COBRA to run out. Our guide on COBRA vs. Medicare at 65 walks through that timing in more detail.
Fewer Than 20 Employees: Medicare Usually Needs to Start at 65
Smaller employer plans work differently. If the company has fewer than 20 employees, Medicare typically becomes the primary payer once you turn 65, and many small-group plans are written to pay only as a secondary payer after that. In practice, that means your employer’s plan may not cover much if you haven’t enrolled in Part A and Part B, even though you’re still an active employee. Confirm this directly with your benefits administrator well before your Initial Enrollment Period ends. Our 7-month Initial Enrollment Period guide covers exactly when that window opens and closes. So, again, don’t delay Medicare Part B if your work has less than 20 people EVEN if you stay on the job and keep your work coverage.
📌 Note: The 20-employee threshold is a federal rule, but how individual plans apply it can vary. Some smaller employers voluntarily coordinate benefits differently. Always confirm with HR rather than assuming.
Should You Take Part A Even If You’re Still Working?
Most people qualify for premium-free Part A, so there’s rarely a downside to signing up for it at 65 even while you keep working and stay on your employer’s group plan for Part B purposes. Part A can help pick up hospital costs your job coverage doesn’t fully cover, and it costs nothing extra if you’ve paid Medicare taxes for at least 10 years.
💡 Tip: The one exception is a Health Savings Account. If you’re still contributing to an HSA, signing up for Part A can trigger a tax problem, because Part A enrollment is backdated up to 6 months and you can’t contribute to an HSA once Part A starts. Senior65 doesn’t advise on HSA tax rules, so check with your HR department or tax advisor before enrolling in Part A if you’re still funding an HSA.
What Happens When You Stop Working or Leave the Plan?
Once your job or your group coverage ends, your 8-month Special Enrollment Period for Part B starts immediately, whether or not you elect COBRA. Sign up promptly. The Part B late enrollment penalty adds 10% to the standard premium ($202.90 in 2026) for every full 12 months you could have signed up but didn’t, and it’s typically charged for as long as you have Part B. Our late enrollment penalty breakdown has the full math.
Enrolling in Part B also starts a separate 6-month Medigap (Medicare Supplement, sometimes called Medsupp or Med Sup) Open Enrollment window, during which you can buy any Medigap plan sold in your state with no health questions. It’s worth deciding ahead of time whether Medigap or Medicare Advantage fits you better, since that window doesn’t reopen automatically. Our Medigap vs. Medicare Advantage comparison covers the tradeoffs.
Get a Medigap Quote Enroll in Medigap
If you miss that 6-month Medigap Open Enrollment window , you can still apply for Medigap through medical underwriting at any time of year. Approval is possible even with some pre-existing conditions, though it’s never guaranteed, and our Medigap Underwriting Checker is the fastest way to check if you will be approved.
Get Your Timing Right Before You Decide
The short version: confirm your employer’s size, sign up for premium-free Part A regardless, and calendar the 8-month window the day your job coverage ends. For a broader overview of how the pieces fit together, see our guide for those new to Medicare, or compare Medigap options on our Medigap overview page. Getting a Medigap quote takes less than 30 seconds once you’re ready to compare plans.
Our team at Senior65 can help you sort out the timing questions specific to your job and state, and there’s no cost to talk it through. We receive commissions directly from insurance companies. Our prices are the guaranteed lowest allowed by law. Call 800-930-7956 to speak with our team, or get a quote online whenever you’re ready.