If you are still working when you turn 65 — or your spouse is working and you are on their plan — you have decisions to make that most people do not know about until it is too late. The rules are counterintuitive, and the mistakes are the permanent kind.
Here is how Medicare and employer coverage actually interact.
Whether Medicare pays first or your employer plan pays first depends on how many employees your employer has.
| Employer size | Who pays first? | What this means for you |
|---|---|---|
| 20 or more employees | Employer plan pays first | You can safely delay Part B while covered |
| Fewer than 20 employees | Medicare pays first | You must enroll in Part B at 65 or face gaps and penalties |
For employees at large companies (20+), your employer plan is still your primary insurance at 65. Medicare would be secondary. Since you are already paying for full employer coverage, many people in this situation choose to delay Part B enrollment — and they can do so without penalty, as long as they enroll within the Special Enrollment Period when their employer coverage ends.
For employees at small companies (fewer than 20), Medicare becomes your primary insurance at 65 whether you enroll or not. If you do not enroll in Part B, your employer plan may refuse to pay for things that Medicare would have covered, leaving you exposed. Enroll in Part B on time.
Part A (hospital insurance) is premium-free for most people who have worked 40+ quarters (10 years). There is almost no reason not to enroll in Part A at 65, even if you are still working. The only exception: if you have a Health Savings Account (HSA) and want to keep contributing to it. Medicare enrollment — even just Part A — disqualifies you from making new HSA contributions. If you are still contributing to an HSA and want to continue, hold off on Part A enrollment and talk to your benefits administrator.
Cannot contribute to an HSA once you are enrolled in any part of Medicare. If your HSA contributions are valuable to you, plan for when you will stop contributing and when you will enroll.
This is the mistake that costs people the most. When you leave a job and take COBRA coverage, you might assume you still have valid employer coverage that protects your Medicare enrollment window. You do not. COBRA is continuation coverage — it is based on a job you already left. It does not count as current employer coverage for Medicare's purposes.
If you leave your job at 65 and take COBRA instead of enrolling in Medicare Part B, your Special Enrollment Period (8 months) starts counting down the day you leave the job — not the day your COBRA runs out. If you let the 8 months pass while on COBRA, you have missed your window.
Retiree health benefits from a former employer also do not count as qualifying current employer coverage. If you retire before 65 and bridge to Medicare using retiree benefits, that is fine — but it does not delay your Part B enrollment window. You need to enroll in Part B at 65 on your regular timeline.
If you are on your spouse's employer health plan, the same size rule applies. If their employer has 20 or more employees, you can safely delay Part B. If their employer has fewer than 20, Medicare becomes primary — which means enrolling in Part B at 65 is generally required under this rule to avoid coverage gaps. And when your spouse retires or loses their job — whenever their employer coverage ends — your 8-month Special Enrollment Period starts for you too.
Reminder: Ready at 65 is not affiliated with Medicare, CMS, the SSA, or any government agency. This article is general educational information, not financial, legal, medical, or insurance advice. Rules, costs, and deadlines change — verify current details at Medicare.gov or by calling 1-800-MEDICARE before making any decision. Full disclaimer →