The 20-employee rule
The single most important factor in Medicare coordination is employer size:
20 or more employees
Your employer plan pays first (primary), Medicare pays second. You can safely delay Part B enrollment without penalty while you have this coverage.
Fewer than 20 employees
Medicare pays first (primary), your employer plan pays second. You should enroll in Medicare at 65 — if you don't, your employer plan may not cover costs that Medicare would have paid.
When to enroll in Medicare
If you work for a large employer (20+), you have flexibility. Many people enroll in premium-free Part A at 65 (since it's free for most) and delay Part B until they retire or lose employer coverage. This saves you the $202.90/month Part B premium while you're still covered at work.
However, if you have an HSA, enrolling in Part A has consequences — see our HSA + Medicare guide.
The COBRA trap
Critical warning: COBRA continuation coverage does NOT count as "employer coverage" for Medicare purposes. If you're offered COBRA when you leave your job at 65+, you must still enroll in Medicare during your 8-month Special Enrollment Period. Relying on COBRA alone after 65 can leave you with a permanent Part B penalty and gaps in coverage.
Retiree health plans
Some Colorado employers offer retiree health benefits. These plans typically require you to enroll in Medicare Parts A and B, then wrap around Medicare as secondary coverage. If your employer offers retiree coverage, enroll in Medicare at 65 — the retiree plan is designed to supplement Medicare, not replace it.
What to do before you retire
- Confirm your employer size (20+ or fewer than 20 employees)
- Contact your HR department to understand how your plan coordinates with Medicare
- If you have an HSA, plan your contribution stop date (6 months before Part A)
- Talk to a licensed Medicare agent 6 months before your planned retirement date