Medicare one-oh-one. Lesson five — you are sixty-five and you are still working. Good. A lot of people are.

But this is the lesson where I need you to pay close attention, because this is where the expensive mistakes live.

There is one question underneath all of this. Does the coverage you already have count? If it counts, you can wait on parts of Medicare with no penalty. If it does not count, and you wait — you are building a penalty right now without knowing it.

The word for coverage that counts is creditable. Write that word down. It is the word to use when you call somebody.

Now — the piece almost nobody knows. The size of the employer changes the answer. If your employer has twenty or more employees, your group health plan generally pays first, and Medicare pays second. In that situation, many people can delay Part B without a penalty while they keep working.

But if your employer has fewer than twenty employees, it flips. Medicare generally pays first, and the group plan pays second.

Think about what that means. If Medicare is supposed to be paying first, and you have not signed up for Medicare — there may be nobody paying first. That is a bill you find out about at the worst possible moment.

So if you work for a small shop — a family business, a small office, a crew of a dozen — this is not a detail. This is the whole ballgame. Ask somebody. Today.

And now the one that catches good, careful people every single year. If you have a Health Savings Account through work — an H-S-A — listen to this part twice.

Once you enrol in Medicare, including Part A, you can no longer contribute to that H-S-A.

Here is why that is a trap and not just a rule. When you sign up for Part A after sixty-five, Medicare can make that coverage retroactive — backdated as much as six months, though never earlier than the month you turned sixty-five.

So somebody signs up in the fall, Part A gets backdated into the spring, and they have been putting money in that account for months during a stretch they were no longer allowed to. They did not cheat. They did not know. And it can cost them at tax time.

If you have a Health Savings Account and you are near sixty-five, talk to somebody before you touch Medicare. Not after.

Last piece. What happens when the job finally ends. When your employment or that group coverage ends, a special window opens — generally eight months — to sign up for Part B without a late penalty.

But there is a catch here too, and it is a big one. COBRA is not the same as working coverage. Neither is retiree coverage. People assume COBRA holds their place in line. It does not. The clock generally starts when the job ends, not when the COBRA runs out.

That misunderstanding has cost people penalties they will pay for life.

So here is lesson five, honestly. This is the one part of Medicare where I will not give you a rule that fits everybody, because there is not one. How big is the employer. Is the coverage creditable. Do you have a Health Savings Account. When exactly does the job end.

Four questions, and the answers decide thousands of dollars. If you are still working at sixty-five — do not guess. Ask. That is Medicare one-oh-one. There are eleven more of these, each about as short as this one, and they are all right here.