Turning 65 does not automatically mean you have to drop your work insurance, nor does Medicare simply take a back seat without any decisions to make. The right answer depends on a factor many people overlook: the size of the company you work for. This guide explains how Medicare and employer coverage interact, how to avoid penalties when you eventually leave that job, and when it makes sense to consider Medicare Advantage or Medigap instead of keeping the group plan.
The 20-employee rule: who pays first
The most important factor is how many employees your company has. If your employer has 20 or more employees, its group health plan is generally the primary payer and Medicare pays second, which lets you delay Part B without penalty as long as you keep that coverage active. If your employer has fewer than 20 employees, the rules flip: Medicare usually becomes the primary payer at 65, and your employer plan pays second, or may even stop covering certain services if you do not have Medicare active.
This rule does not depend on your opinion or your manager's; it depends on how the employer's health plan is structured and reported. Confirming this with the HR department, rather than assuming it, is the first step before deciding what to do about Medicare at 65.
Enrolling in Part A only versus Part A and B together
If you qualify to delay Part B because you have coverage through an employer with 20 or more employees, many people still enroll in Part A, since it carries no premium if you paid enough into Social Security during your working years. Part A mainly covers hospital stays, so having it active usually does not interfere with your work plan or create an added cost.
There is one important exception: if you or your employer contribute to a health savings account (HSA) paired with a high-deductible plan, enrolling in Part A stops you from making new HSA contributions going forward, regardless of whether you also enroll in Part B. If you rely on those contributions, it is worth reviewing the exact timing of your Part A enrollment before moving ahead.
Availability, cost, benefits, and eligibility vary by person, product, contract, and insurance company, as well as by your service area. Guarantees are subject to the claims-paying ability of the issuing insurance company. Karla Arámburo is not affiliated with or endorsed by Medicare or the federal government.
The Special Enrollment Period when you leave your job
When you finally leave your job, or your employer stops offering health coverage, a Special Enrollment Period (SEP) opens up: you generally have eight months, counted from the month your employment or your coverage ends, whichever comes first, to enroll in Part B without a penalty. That same window usually applies to enrolling in a Part D plan if your work drug coverage was creditable.
A common mistake is confusing the SEP with COBRA: continuing on COBRA after leaving your job does not extend your Medicare SEP. The eight-month clock starts when your active employment or employment-based coverage ends, not when COBRA ends, so waiting until COBRA runs out to enroll in Medicare can result in a permanent late enrollment penalty.
How benefits coordinate between your employer plan and Medicare
When you have both coverages active, coordination of benefits determines who pays first and who pays second for each medical service. If your employer is the primary payer (companies with 20 or more employees), the claim is processed with your work plan first, and Medicare can cover part of what remains. If Medicare is the primary payer (companies with fewer than 20 employees), the order flips, and not having Medicare active in that scenario can leave you with unexpected medical bills that your employer plan no longer fully covers.
That is why confirming your employer's size and the correct payment order, before you need costly medical care, avoids financial surprises at a difficult moment.
When it makes sense to switch to Medicare Advantage or Medigap
Keeping your work plan is not always the cheapest option or the one with the best coverage, even while you are still employed. It is worth comparing whether a Medicare Advantage plan or a Medigap policy, paired with Original Medicare, would give you a lower premium, a broader network, or extra benefits your work plan does not offer, especially if your employer recently raised deductibles or narrowed its provider network.
If you already know you will retire soon, it also helps to plan ahead: review our Medicare enrollment dates 2026 guide to know your exact windows, and compare Medicare Advantage against Original Medicare along with our Medigap guide to understand your options once you leave your employer plan.
Bilingual help for your specific situation
I am Karla Arámburo, an independent, bilingual insurance agent licensed in California. I help people who are still working past 65 in cities like San Diego confirm their employer's size, review whether it makes sense to delay Part B, and prepare their Special Enrollment Period well ahead of when they eventually retire. This guide is general education, not legal, tax, or specific employer-benefits advice; the rules for each work plan can vary, so it is always worth confirming the details with HR and with Medicare directly before making a decision.
You can call or text (619) 321-8733 to schedule a free consultation and review your particular case, in Spanish or English.
Frequently asked questions
Do I have to enroll in Medicare at 65 if I am still working?
Not always. If your employer has 20 or more employees and its group health plan counts as creditable coverage, you can generally delay Part B without penalty while you keep that coverage. If your employer has fewer than 20 employees, the rules change and Medicare usually becomes your primary coverage at 65.
Should I enroll in at least Part A even if I am still working?
For most people, yes, because Part A carries no premium if you have enough Social Security work credits. However, if you contribute to a health savings account (HSA), enrolling in Part A can stop your ability to make future HSA contributions, so it is worth reviewing before you sign up.
What happens if my employer has fewer than 20 employees?
In that case, Medicare generally becomes your primary coverage at 65, and your employer's plan becomes secondary. If you do not enroll in Part B on time in this situation, your employer plan may pay far less than expected, and you could face a late enrollment penalty.
How long is my Special Enrollment Period when I leave my job?
You generally have eight months starting the month your employment or your employer coverage ends, whichever happens first, to enroll in Part B without a penalty. That same eight-month window generally applies to Part D if your drug coverage was creditable.
Can my employer require me to enroll in Medicare at 65?
If your employer has 20 or more employees, it generally cannot require you to enroll in Medicare as a condition of keeping the group plan. If it has fewer than 20 employees, the rules are different, and it is worth confirming with HR how their plan coordinates with Medicare.
When does it make sense to switch from my work plan to Medicare Advantage or Medigap?
It depends on your budget, your doctors, and how soon you plan to retire. Some people prefer to keep their work plan as long as it stays competitive on cost and network, while others find a Medicare Advantage or Medigap plan gives them better coverage or a lower cost. Comparing both options side by side, instead of assuming the work plan is always better, avoids surprises.
Can you help me decide at no cost?
Yes. I review your employer's size, your current coverage, your enrollment dates, and your retirement timeline to help you decide what to do about Medicare while you are still working, in Spanish or English, at no cost to you.