Working Past 65: Do You Need Medicare If You Have Employer Insurance?

You turn 65 in four months. You're still working, you like your employer plan well enough, and the mail has turned into a blizzard: Medicare guides, Advantage plan pitches, letters warning you about penalties. HR says "ask Medicare." Medicare's website says "ask your employer." Meanwhile you have an HSA quietly auto-contributing every paycheck. This fork is genuinely consequential: three of the most expensive, least reversible mistakes in all of Medicare live right here, and every one of them is avoidable with about twenty minutes of clarity.

The 20-employee rule: who pays first

20 or more employees: your employer plan stays primary while you (or your spouse) keep working, and Medicare rules let you delay Part B penalty-free for as long as that current-employment coverage lasts. Many people in this group take premium-free Part A at 65, skip Part B (and its monthly premium) entirely, and enroll later using the Special Enrollment Period covered below. That's a legitimate, penalty-free path, with one big exception for HSA users, next section.

Under 20 employees: the order flips. Medicare becomes primary and the employer plan pays secondary, meaning it calculates its share as if Medicare paid first, whether or not you actually enrolled. Skip Part B here and you've built a coverage hole: Medicare pays nothing because you're not enrolled, and the employer plan may pay only the small slice it would owe after Medicare. People discover this after a surgery, staring at a five-figure balance both payers deny. If your employer is under 20 employees, take Part A and Part B at 65. Small-employer plans can also legally require you to enroll; ask HR directly whether the plan assumes Medicare enrollment at 65.

The HSA trap: stop contributing before you enroll

Under IRS rules (Publication 969), you cannot contribute to a health savings account in any month you're enrolled in any part of Medicare, including premium-free Part A. The part that catches people: when you enroll in Part A after 65 (or file for Social Security, which triggers automatic Part A), the coverage backdates up to 6 months, though never earlier than your 65th-birthday month. Contributions made during those backdated months become excess contributions, taxed with a 6% excise until removed. The move is simple once you know it: stop HSA contributions (yours and your employer's) about 6 months before you enroll in Medicare or file for Social Security. You keep the account and spend it tax-free forever, including on Medicare premiums; only new contributions end. If you already over-contributed, ask your HSA custodian about withdrawing the excess before the tax deadline.

The COBRA trap: it does not let you delay Part B

The penalty-free delay only works while coverage comes from current employment. COBRA is, by definition, coverage after employment ends, so it doesn't count, and neither does retiree coverage. People leave a job at 66, take 18 months of COBRA because it's familiar, and walk into two problems at once: a lifetime Part B late-enrollment penalty (an extra 10% of the premium for each full year they should have been enrolled) and a wait for coverage after their COBRA ends. Worse, once you're 65, COBRA can coordinate as if Medicare pays first, so it may cover very little on its own. The rule of thumb: your 8-month Part B clock starts when employment ends, and COBRA does not pause it. If you take COBRA at all after 65, enroll in Part B anyway, inside that window.

Two smaller decisions: Part A at 65, and drug coverage

• Part A now or later? Most people who worked 10 years take Part A at 65 even while employed: it's premium-free and can pick up some hospital costs behind the employer plan. The main reason to delay is the HSA rule above, since even Part A alone ends contribution eligibility. If you're funding an HSA, delay Part A and don't file for Social Security until you're ready to stop contributing.
• Part D: check the word "creditable." You can skip a Medicare drug plan penalty-free only if your employer drug coverage is creditable (at least as good as standard Part D). Your plan must tell you each year in a creditable-coverage notice, usually mailed in September. Keep that letter. When the coverage ends, you have 63 days to join a Part D plan before a late penalty starts building.
• Working past 65 with a marketplace plan instead? Different story: marketplace coverage doesn't let you delay penalty-free, and your premium subsidies end once Medicare eligibility begins. That fork is covered in the enrollment deadlines guide.

When the job ends: the 8-month SEP, step by step

1. Mark the clock. Your Special Enrollment Period for Part B runs 8 months from the month employment or the employer coverage ends, whichever comes first. Enrolling within it means no penalty and no waiting for a general enrollment window.
2. File two forms with Social Security: CMS-40B (your Part B application) and CMS-L564 (Request for Employment Information), which your employer completes to prove you had job-based coverage since 65. Ask HR for the L564 before your last day; chasing a former employer for paperwork is nobody's favorite month.
3. Line up drug coverage inside 63 days of losing creditable employer coverage: a standalone Part D plan, or a Medicare Advantage plan that includes drugs.
4. Decide the Medigap question early. Your one-time guaranteed-issue window for a Medigap policy runs 6 months from when Part B starts. The Medigap vs Medicare Advantage guide walks through that choice; it's much harder to switch later, so treat it as part of this same decision.
5. If you're keeping any secondary coverage (retiree plan, spouse's plan), learn how it coordinates with Medicare before the first big claim. The secondary insurance guide explains who pays what.