The 20-employee rule
If your (or your spouse's) employer has 20 or more employees, the group plan is usually primary and Medicare is secondary. You can safely delay Part B until you retire and use the 8-month Part B Special Enrollment Period to enroll without penalty.
If the employer has fewer than 20 employees, Medicare becomes primary at 65. The group plan only pays after Medicare does — which means you generally need to enroll in Part B on time or face penalties and gaps.
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COBRA and retiree coverage
- COBRA is NOT creditable coverage for Part B. Going on COBRA at 65 without enrolling in Part B usually triggers a late-enrollment penalty.
- Retiree health plans almost always require you to enroll in Medicare A and B — they pay secondary.
- HSA contributions stop the month you enroll in any part of Medicare (including premium-free Part A).
- Spouse on the employer plan may need her or his own Medicare strategy.
When you retire — the 8-month SEP
Once your employer coverage ends (or you stop working — whichever is first), you have 8 months to enroll in Part B without penalty. You also have a 2-month window to enroll in a Part D or Medicare Advantage plan. Acting in the first 60 days usually avoids any coverage gap.
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