The short answer
If you delayed Medicare because you were still working and had group health coverage, you get a special enrollment period (SEP) to sign up for Part B without a late penalty once that employment or coverage ends. The window is eight months, and it starts the month after your employment ends or your group coverage ends, whichever comes first.
Used correctly, this SEP is how millions of people work past 65 and then move onto Medicare smoothly. Used incorrectly — usually by leaning on COBRA — it's how people accidentally trigger a lifetime penalty.
How the 8-month window works
The eight months begin the month after your employment ends or your employer group coverage ends. During that window you can enroll in Part B (and Part A if you hadn't) without the late enrollment penalty, and coverage starts based on when you sign up. It's a clean, penalty-free on-ramp to Medicare after a working career.
The mistake to avoid is waiting. If you let all eight months pass without enrolling, you can fall out of the SEP and into a general enrollment period, with a penalty and a delayed start. Don't sit on it.
The COBRA trap
Here's the part that catches people. COBRA is not considered active employer coverage for Medicare purposes. So if you retire, go on COBRA, and assume that keeps your Medicare enrollment window open, you're mistaken — your 8-month SEP clock started when your active employment ended, not when COBRA ends. People burn through the window on COBRA and then face a Part B penalty and a coverage gap.
If you're retiring and weighing COBRA versus Medicare, treat the two as separate decisions and enroll in Medicare within the SEP. Our guide to whether you need Part B with employer coverage covers the timing, and our pre-Medicare coverage overview covers bridging options.
Frequently Asked Questions
How long is the Medicare special enrollment period after employer coverage ends?
Eight months. It starts the month after your employment ends or your group health coverage ends, whichever is first, and lets you enroll in Part B without a late penalty.
Does COBRA extend my Medicare enrollment window?
No. COBRA doesn't count as active employer coverage for Medicare. Your 8-month special enrollment period starts when your active employment ends, not when COBRA ends.
What happens if I miss the 8-month window?
You can fall into a general enrollment period, which comes with a Part B late penalty and a delayed coverage start date. Enroll within the SEP to avoid both.
When should I enroll if I'm retiring at 66?
Enroll in Part B during your 8-month special enrollment period, ideally timed so Medicare begins the month your employer coverage ends, to avoid any gap.
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