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Plenty of people keep working past 65, and the health insurance math changes the day you become Medicare-eligible, even if nothing else about your job changes. The two questions that matter are which coverage pays first and what happens to your HSA.
Who pays first depends on employer size
If your employer has 20 or more employees, the group health plan pays first and Medicare pays second. You can delay Parts A and B without penalty while you have that coverage, and many people do, since the employer plan is often better or cheaper than Medicare plus a supplement.
If your employer has fewer than 20 employees, Medicare pays first once you turn 65. That flips the usual logic. The small-group plan may refuse to pay claims it considers Medicare’s responsibility, which means delaying Part B at a small employer can leave you effectively uninsured. If you work for a small company, talk to your benefits administrator before your 65th birthday and get the answer in writing.
The HSA trap
This is the one that costs people real money. You cannot contribute to a health savings account once you are enrolled in Medicare, including Part A. And Medicare backdates Part A coverage up to six months when you enroll after age 65. So if you sign up for Medicare at 67, your Part A is treated as starting six months earlier, and any HSA contributions you made in those six months become excess contributions, subject to a 6 percent excise tax.
The safe move: stop HSA contributions six months before you plan to enroll in Medicare if you are enrolling late. If your employer plan is a high-deductible plan and you want to keep the HSA going, you can delay all of Medicare, Parts A and B, while you have qualifying employer coverage. But you have to delay Part A too, which means not filing for Social Security retirement benefits either, since that triggers automatic Part A enrollment.
When you finally retire
Losing employer coverage gives you a special enrollment period: eight months to sign up for Part B without penalty, starting when employment or the group coverage ends. Do not confuse this with COBRA. Electing COBRA does not extend the special enrollment period, and COBRA itself does not count as coverage that lets you delay Part B penalty-free.
Your Medigap open enrollment period, the six-month window when insurers cannot use medical underwriting, starts when you enroll in Part B. If you delayed Part B for years while working, that six-month window starts at retirement, not at 65. That is good news, but do not let it slide. Once it closes, switching or buying Medigap in most states means answering health questions.
What to do before your birthday
First, find out whether your employer has 20 or more employees. Second, decide whether to keep contributing to an HSA, and if so, plan your Medicare enrollment date around the six-month backdating rule. Third, confirm in writing that your employer drug coverage is creditable, so you avoid the Part D late penalty later.
If you are weighing retirement a few years early instead, the early retiree bridging guide covers COBRA, marketplace plans, and Medicaid for the gap years. And for the basics of what you are signing up for, see Medicare parts A, B, C, and D explained.
