Delay Medicare at the wrong moment and you can pay a penalty every month for the rest of your life. Enroll at the wrong moment and you can owe the IRS money back on an HSA you thought was yours to fund.
Both traps are avoidable, and which one you are near comes down to a single number almost nobody asks about: how many people does your employer have? Federal rules draw a hard line at 20 employees. Which side of it you are on decides whether delaying Medicare is safe or whether it quietly leaves you with no primary coverage at all.
This is an education piece, not a sales pitch. We name no plans and compare no benefits. We walk the rules, the deadlines and the two traps that cost real money, then tell you what to do with the answer.
The 20-employee line decides almost everything
Twenty or more employees, your plan pays first and you can usually delay Part B. Under twenty, Medicare pays first and delaying is dangerous.
Under the Medicare Secondary Payer rules, if your employer has at least 20 employees and your coverage comes from current employment, the group plan pays first and Medicare is secondary. That is the situation where delaying Part B is normally safe, because you already have primary coverage.
If your employer has fewer than 20 employees, the order flips. Medicare becomes the primary payer. That is the trap, because nothing about your card or your paperwork changes to tell you. Your group plan quietly becomes the secondary payer, and if you have not enrolled in Medicare, the primary payer simply does not exist. You can be paying premiums and still be exposed on a large claim.
Who pays first at 65 if you are still working
| Employer size | Who pays first, and what that means |
|---|---|
| 20 or more employees | Your group plan pays first. Medicare is secondary, so delaying Part B is normally safe. |
| Fewer than 20 | Medicare pays first. Your group plan pays second, so not enrolling leaves no primary payer at all. |
Scroll the table sideways to see every column.
If you delay, the clock that matters starts when the job ends
The special enrollment period runs 8 months from the end of employment or the end of the coverage, whichever comes first.
If you legitimately delayed Part B because you had employer coverage from current employment, you get a special enrollment period rather than a penalty. It starts the first month after your initial enrollment period ends, and it runs for 8 months after the group health plan coverage or the employment ends, whichever happens first.
Read that last clause twice, because it is where people get caught. The clock starts at whichever comes first. If you retire in March and your employer keeps you on the plan through June, the eight months are measured from the earlier date, not the later one.
- COBRA is not current employment. Electing COBRA after you stop working does not extend this window. The clock already started when the employment ended.
- Retiree coverage is not current employment either. It is a different thing from the active plan you had while working, and it does not preserve the special enrollment period.
- It does not apply if your Medicare eligibility is based on end-stage renal disease. That situation follows separate coordination rules.
- Coverage generally starts the first month after you sign up. If you enroll while still working, or within the first full month after employer coverage ends, you can ask to delay the Part B start date by up to three months.
The penalty is not a late fee, it is a permanent surcharge
Ten percent for every full year you could have had Part B and did not, for as long as you have Part B.
People hear penalty and picture a one-time charge. Medicare's Part B penalty is 10% added to your monthly premium for each full 12-month period you could have signed up and did not, and for most people it lasts as long as they have Part B.
Medicare's own 2026 example: someone who waited two full years pays a 20% penalty on top of the standard premium. At the 2026 standard premium of $202.90, that is an extra $40.58 every month, producing a premium of $243.50. That is roughly $487 a year, every year, forever, for a two-year delay.
If you contribute to an HSA, read this before you enroll in anything
Medicare enrollment ends HSA contributions, and Part A can be backdated six months.
This is the trap that costs the most money and gets mentioned the least. Under IRS rules, your HSA contribution limit drops to zero beginning with the first month you are enrolled in Medicare. That includes Part A on its own. Part A is premium-free for most people and feels like it costs nothing to take, which is exactly why it catches people out.
The mechanism is retroactivity. Sign up for Part A after 65, or apply for Social Security, and coverage can start up to six months earlier than the date you signed up. It will never start before the month you turned 65. The IRS is explicit about what that does. If your enrollment is backdated, contributions you made during that retroactive period are treated as excess contributions.
If you are not sure how a high-deductible plan and an HSA fit together in the first place, we explain the mechanics in our plain guide to HMO, PPO and HDHP plans.
- Claiming Social Security enrolls you in Part A automatically. If you are 65 or older and claim benefits, Part A comes with it. That alone ends HSA eligibility.
- Plan the stop date backward. If you intend to keep contributing to an HSA while working, the common approach is to stop contributions six months before you expect to enroll in Part A or claim Social Security.
- Your employer's contribution counts too. Money the employer puts in is still a contribution to your account, so it is caught by the same limit.
- You can still spend the balance. Losing the ability to contribute is not the same as losing the account. Existing funds remain available for qualified expenses.
The order you do things in changes the outcome
Claiming Social Security, enrolling in Part A and retiring each start a different clock. Doing them in the wrong order costs money.
These decisions are not independent, and that is what catches people. Each one starts a different clock, and the clocks interact.
- You turn 65Your initial enrollment period runs for seven months, from three months before the month you turn 65 to three months after it.
- You claim Social SecurityAt 65 or older this enrolls you in Part A automatically, and HSA contributions have to stop that same month.
- Your employer coverage endsThe eight-month Part B window starts here, or at the end of the job, whichever comes first. COBRA does not extend it.
- You sign up for Part ACoverage can backdate up to six months, reaching into months you may already have funded an HSA. It never starts before the month you turned 65.
None of those rules is complicated on its own. The mistakes happen in the gaps between them, usually because the person handling one piece never talks to the person handling the next.