It depends on the size of your employer and the type of coverage you have. Getting this wrong can trigger a permanent late-enrollment penalty added to your Part B premium for the rest of your life — plus gaps in coverage. So get a written confirmation in hand before deciding.
If your employer has 20+ employees AND you're actively working (not retired):
- Your employer plan is primary, Medicare is secondary
- You can usually delay Part B and Part D without penalty
- Most people sign up for Part A only at 65 (it's free if you've worked 10+ years), keep their employer coverage as primary
- When you eventually leave the job, an 8-month Special Enrollment Period (SEP) opens for Part B and Part D — no penalty
- Caveat: if you contribute to an HSA, you must stop contributions when you enroll in any part of Medicare (including Part A). HSA owners often delay even Part A.
If your employer has fewer than 20 employees:
- Medicare is primary at age 65, employer is secondary
- You almost certainly should enroll in Part A and Part B at 65 — otherwise the employer plan may pay nothing for services Medicare would have covered
- Confirm with your HR or benefits administrator in writing how the small-employer plan coordinates with Medicare
If you have COBRA, retiree coverage, or an ACA Marketplace plan:
- These do NOT count as creditable coverage for delaying Medicare
- You should enroll in Part A and Part B during your IEP at 65 to avoid penalty
- COBRA and ACA premiums often drop or change when Medicare becomes available; you may want to drop them and switch to Medicare
Part D specifically: Even if you delay Part B, you must have creditable drug coverage (your employer plan's coverage must be at least as good as Medicare Part D). Your employer must give you an annual notice each fall stating whether your drug coverage is creditable. Save these notices — Social Security may ask for proof later.
The penalty if you get it wrong: 10% added to your Part B premium for every 12-month period you could have had Part B but didn't, for the rest of your life. So delaying Part B for 5 years = 50% premium surcharge forever. Same logic for Part D (1% per uncovered month).
What to do next: Call (866) 534-1886 — we walk through your specific employer situation, confirm whether your coverage is creditable, and time your Medicare enrollment to avoid both gaps and penalties. Free.