Turning 65 doesn't mean you have to leave your employer plan. But it does start a few clocks, and the penalties for guessing wrong are permanent. Here's the plain version.
The size of your employer matters
| Employer size | Who pays first | What that means |
|---|
| 20+ employees | Your group plan is primary; Medicare is secondary | You can generally delay Part B without penalty while covered by the group plan. Many people enroll in Part A (usually free) and wait on B. |
| Under 20 employees | Medicare is primary; the group plan is secondary | You generally need Part A and B at 65 — the group plan may pay very little until you do. Delaying B here is the expensive mistake. |
The HSA catch
Once you enroll in any part of Medicare — including "free" Part A — you generally can't contribute to an HSA anymore. And if you delay Medicare past 65 and then apply for Social Security, Part A enrollment is typically backdated up to six months. Stop HSA contributions six months before you plan to enroll to avoid an excess-contribution penalty. The money you already have is still yours to spend.
The clocks
- Initial Enrollment Period — the seven months around your 65th birthday (three before, the birth month, three after).
- Special Enrollment Period — if you delayed because of employer coverage (20+ employer), you generally get eight months after that coverage ends to enroll in Part B with no penalty.
- Part D — as long as your employer drug coverage is "creditable" (your plan sends a notice each year saying so), you can delay without penalty. Keep those notices.
- Medigap Open Enrollment — six months from when you enroll in Part B. Inside it, carriers generally can't underwrite you. Outside it, they often can.
Common paths
- Keep working, keep the group plan (20+ employer): enroll in Part A, delay B and D, stop HSA contributions before enrolling in A.
- Keep working at a small employer: enroll in A and B at 65; compare whether keeping the group plan as secondary is worth its premium.
- Spouse on your plan: their coverage depends on your plan, not your Medicare. If you leave the plan for Medicare, they'll need their own coverage — COBRA, Marketplace, or their own employer.
Who to call
Timing and penalty questions: Social Security (1-800-772-1213) or your free State Health Insurance Assistance Program (SHIP). How your group plan coordinates: HR or the carrier. Don't rely on the pharmacy or a friend — the rules are specific to your employer's size and your plan.