Not everyone stops working at 65 anymore. If you're still on the job with employer health insurance, you don't automatically have to jump into Medicare — but whether you should wait depends on one detail most people don't know to check: the size of your employer.

The rule that changes everything: how many employees?

If you're still working and have insurance through your employer, Medicare is either your primary or secondary coverage depending on your company's size — and that determines whether it's safe to delay.

  • If your employer has 20 or more employees: Your employer's insurance pays first, and Medicare pays second. In this case, you can usually delay Part B without penalty and stay on your employer plan.
  • If your employer has fewer than 20 employees: Medicare pays first, and your employer plan is secondary. This is the critical one — if you delay enrolling in Medicare here, you could end up with gaps in coverage and late penalties, because your small-employer plan expects Medicare to be your primary coverage.

That single distinction — 20+ employees or fewer — is the first thing to nail down. But it's a starting point, not the whole answer: whether it's right for you to delay Medicare depends on your specific employer coverage and situation. Because of that, it's important to confirm how your employer plan coordinates with Medicare before deciding to delay enrollment. Your employer's HR or benefits department can tell you whether your coverage is based on current employment and how it works with Medicare.

Should you take Part A anyway?

Even if you delay Part B while working, many people still enroll in premium-free Part A at 65, since it costs nothing if you qualify based on work history and can act as secondary hospital coverage. There's one exception worth knowing: if you contribute to a Health Savings Account (HSA), enrolling in any part of Medicare (including Part A) means you can no longer contribute to your HSA. So if you're actively saving in an HSA, that's a specific thing to weigh before signing up for Part A.

Your penalty-free window when you finally stop working

Here's the reassuring part: if you have qualifying employer coverage (from an employer with 20+ employees) and delay Medicare, you don't lose your chance to enroll later without penalty. When that employment or coverage ends, you get a Special Enrollment Period — 8 months to enroll in Medicare without any late penalty. So working past 65 doesn't trap you; it just changes your timing.

Watch out for these coverage traps

A few situations catch people off guard:

  • COBRA coverage generally does not extend your Part B Special Enrollment Period. That Special Enrollment Period is available based on current-employment group health plan coverage — not COBRA — so confirm your enrollment requirements before relying on it. On top of that, once you're Medicare-eligible, Medicare becomes your primary insurance and COBRA only pays second, which can get expensive. In most cases, enrolling in Medicare is the better move than relying on COBRA.
  • Retiree health benefits usually make Medicare primary. If you retire and have retiree coverage from a former employer, Medicare typically pays first and the retiree plan is secondary. Some retiree plans require you to enroll in Parts A and B — and if you don't, they won't pay anything. Always confirm with the benefits office.
  • Marketplace (ACA) plans: Once you're eligible for Medicare, you're no longer eligible for ACA Marketplace subsidies.

The bottom line

Working past 65 gives you options — but the right move depends entirely on your situation: your employer's size, whether you have an HSA, and what happens when you eventually retire. The mistake is assuming you can just “deal with it later” without checking the rules first, because a wrong guess here can cost you penalties for life.

This is one of the most common situations where a quick conversation saves people real money and headaches — because the answer genuinely is different for a person at a 15-person company than at a 500-person company.