Most of the expensive mistakes I see in retirement planning as a CFP® are subtle. This one is not.
A Medicare enrollment trap springs most often on the very people who feel least worried about it: Those working past 65 with what they assume is perfectly good employer coverage.
But a hard Medicare deadline with a permanent penalty attached can snare people who did everything else right.
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Here’s how it happens, why it costs real money and what to do about it.
The setup
When you turn 65, you become eligible for Medicare. Most people know that. What trips them up is the assumption that because they’re still working and covered by an employer health plan, they can ignore Medicare until they retire.
Sometimes that is true. Often, it’s not, and the difference comes down to the size of the employer and the type of coverage you have.
If you work for a larger employer, generally one with 20 or more employees, your group plan can remain your primary coverage, and you can delay Medicare Part B without penalty under a special enrollment period.
But if you work for a smaller employer, Medicare might be considered your primary payer once you turn 65, even while you’re still on the company plan.
In that situation, delaying Part B doesn’t just risk a penalty. It can leave you with gaps in coverage, because your employer plan might pay as though Medicare is already covering its share, whether or not you’re enrolled.
That distinction — 20 employees —isn’t something most people think to check. They see, “I have health insurance through work,” and reasonably conclude they’re fine. The rules don’t care how reasonable the assumption was.
The penalty that never goes away
Here’s the part that stings. If you’re required to enroll in Part B and you miss your window, the late enrollment penalty is not a one-time fee.
According to Medicare.gov, the Part B penalty adds 10% to your premium for each full 12-month period you could have had Part B but did not. You pay that surcharge for as long as you have Part B, which for most people means the rest of their lives.
Think about what that means. Delay three years when you should have enrolled, and you’re looking at a 30% premium surcharge, every month, indefinitely. There is no statute of limitations, no point where it falls off. A misunderstanding at 65 becomes a line item you carry into your nineties.
Because that 30% applies to whatever the standard premium happens to be each year, the dollar cost climbs right along with premiums over time. Across a 20-year retirement, a few years of delay can quietly add up to thousands of dollars in surcharges you could have avoided.
I have seen people discover this years later and have no recourse, because the rules were followed exactly as written — just not by them.
The timing windows that matter
Medicare enrollment runs on specific windows, and missing them is what triggers the trouble. Your initial enrollment period is a seven-month stretch around your 65th birthday: the three months before, your birthday month, and the three months after.
If you qualify to delay because of active employer coverage at a large employer, you get a special enrollment period to sign up later without penalty, typically while you’re still covered and for eight months after that coverage ends.
The danger zone is the space between assumptions and rules. People who retire and lose coverage sometimes assume they can enroll whenever they get around to it.
The eight-month clock after employment ends is easy to blow past, especially amid the chaos of leaving a job. Miss it, and you might be stuck waiting for a general enrollment period and carrying the penalty besides.
What to do
A few concrete steps make this entirely manageable.
Well before you turn 65, find out if your employer has more or fewer than 20 employees and confirm in writing how your plan coordinates with Medicare. Your human resources (HR) department or benefits administrator should be able to tell you whether your coverage lets you delay Part B. Do not guess, and do not rely on a coworker’s situation, which might differ from yours.
If you’re at a small employer, treat your 65th birthday as a real deadline and enroll during your initial enrollment period unless you have confirmed you genuinely qualify to wait.
If you’re delaying because of large-employer coverage, mark the eight-month special enrollment period that begins when that coverage ends. Put it somewhere you won’t lose it. The moment you retire or drop the employer plan, the clock starts.
Remember that Part D, prescription drug coverage, has its own late enrollment penalty with similar lifetime consequences. If your employer coverage is not considered creditable drug coverage, the same kind of permanent surcharge can apply.
There is one more wrinkle worth flagging, because it catches a particular group off guard. If you contribute to a health savings account, enrolling in any part of Medicare, including Part A, ends your ability to make new HSA contributions.
Part A is premium-free for most people and many enroll in it automatically without thinking, but doing so while still funding an HSA can create a tax problem. If you work past 65, still contributing to an HSA and plan to delay Medicare, that interaction needs to be on your radar.
It’s a small detail with outsize consequences and is precisely the kind of thing that gets missed when someone assumes Medicare is a single on-off switch rather than a set of separate decisions.
The bigger lesson
What makes this trap so frustrating is that it punishes inaction by people who weren’t being careless, they were just uninformed. There is no investment decision here, no market risk, no judgment call about strategy. It’s purely a matter of knowing the rules and the dates, and acting before a deadline that doesn’t announce itself.
If you’re approaching 65 and still working, don’t let, “I have coverage through my job,” be the end of your analysis. It’s the beginning of a question, not the answer.
Spend an afternoon confirming exactly how your employer plan interacts with Medicare, and put the relevant enrollment windows on your calendar.
It’s one of the rare retirement mistakes that is completely avoidable with a single phone call, and one of the few in which the cost of getting it wrong follows you for the rest of your life.