Working past 65: the Medicare-employer coverage decision tree

Turning 65 while still employed drops you into one of Medicare's most consequential decision windows — and one of its worst-documented. Enroll in Part B you don't need and you'll pay premiums (and possibly IRMAA surcharges) for nothing. Skip Part B you did need and you face a late enrollment penalty that lasts the rest of your life, plus months without coverage. The right answer turns almost entirely on one question nobody thinks to ask: how many employees does your company have?

The 20-employee rule decides everything

For workers 65+ with employer coverage, federal coordination rules split the world in two. At employers with 20 or more employees, the group plan pays primary and Medicare pays secondary — meaning your employer plan works fine on its own, and you can safely delay Part B. At employers with fewer than 20 employees, Medicare pays primary even if you never enrolled — and the group plan can legally refuse to pay the share Medicare 'would have' covered. Small-firm employees who skip Part B can be left functionally uninsured while paying premiums for a plan that now covers almost nothing.

The HSA collision: Part A's six-month lookback

Premium-free Part A seems like an obvious yes — until you remember that any Medicare enrollment ends HSA contribution eligibility. Worse, if you enroll in Medicare after 65, Part A coverage is backdated up to six months (not earlier than your 65th birthday month). Contributions made during that retroactive window become excess contributions subject to a 6% excise tax. And claiming Social Security automatically enrolls you in Part A — you cannot decline it while taking benefits. The planning consequences:

• Still contributing to an HSA past 65? Delay both Part A and Social Security, and stop HSA contributions at least six months before you eventually enroll.
• In your final contribution year, prorate: eligibility is measured monthly, so contributing the full annual limit before a mid-year Medicare start creates excess contributions.
• You can still spend HSA money forever — and after 65, HSA dollars can pay Part B, Part D, and Medicare Advantage premiums tax-free (though not Medigap premiums). Only contributions end.
• Married with a younger spouse? Your Medicare enrollment doesn't end their eligibility — a spouse under 65 on family HDHP coverage can keep contributing to their own HSA, including catch-up.

The COBRA trap: the costliest wrong assumption

The Part B special enrollment period is available while you're covered by a group plan based on current employment — and for 8 months after that employment ends. COBRA is not current employment. Neither is retiree coverage or severance-period coverage. People routinely retire at 66, take 18 months of COBRA, and assume they'll enroll in Medicare when it runs out. Instead they discover: no special enrollment period, a wait for the general enrollment period, and a lifelong Part B penalty of 10% per full 12-month period they went without. On top of that, COBRA typically pays secondary to Medicare after 65 — even if you never enrolled — so the insurer can claw back claims it paid primary. The rule is simple and unforgiving: when active employment ends, the 8-month Part B clock starts, COBRA or not.

Enrolling later: the paperwork that proves your delay was legal

1. 1
   Start 2–3 months before retirement (or the coverage switch)

   Part B applications with employment-based delays are processed manually and take time. Starting early prevents a coverage gap between the group plan ending and Part B starting.

2. 2
   File CMS-40B plus CMS-L564

   The 40B is your Part B application; the L564 is the employer's certification of when you had group coverage based on active employment. Every employer since you turned 65 needs to complete one — chase these before HR contacts go stale.

3. 3
   Pick your Part B start month deliberately

   Align it with the group plan's end date to the month. Also remember Medigap: your 6-month guaranteed-issue Medigap window keys off Part B's effective date — delaying Part B past 65 preserves it, but once Part B starts, that clock runs.

4. 4
   Line up Part D or Medicare Advantage in the same window

   Losing employer coverage triggers a 2-month Part D special enrollment period. Shorter than the Part B window — handle both together.

The bottom line

Working past 65 gives you real leverage — free delay of Part B behind a large employer's plan, extra years of HSA contributions if you keep Medicare entirely at bay, and a clean 8-month runway when you finally retire. The traps are equally real: the under-20-employee primary-payer rule, Part A's six-month lookback colliding with HSA contributions, and the COBRA assumption that has cost thousands of retirees a lifelong penalty. Decide with three facts in hand — employer size, HSA status, and your retirement date — and file the L564 paperwork early. This is one corner of retirement where the difference between optimal and wrong is purely informational.