Updated August 3, 2026. Quick answer: there are really only two decisions, and they are not equally reversible. Decision one is Original Medicare or Medicare Advantage. Decision two is which drug coverage. Decision one has a six-month clock attached that never restarts, so it is the one to take seriously first.
The order that avoids the expensive mistakes
- Confirm when your Part B actually starts. Everything else keys off it, including the Medigap window. If you are still working, this is not automatic — what employer coverage does to the timing.
- Stop contributing to an HSA in time. Medicare enrolment can be backdated, and contributions inside the backdated period create a tax problem. The six-month lookback.
- Make the coverage choice — Original Medicare plus Medigap, or Advantage. This is the one with the one-way door in it.
- If Medigap: buy inside the six-month window. It does not come back.
- Sort drug coverage — and do it even if you take no drugs, because the late penalty is permanent. What it costs.
- Check your income two years back. Part B and Part D premiums are surcharged on it. Why 2026 premiums look at 2024.
If you are coming off a marketplace plan
Turning 65 ends the pre-65 bridge, and the two systems have nothing in common — different rules, different subsidies, different definitions of income. If you have been managing income to hold a subsidy, that constraint ends and a different one begins. The pre-65 side and why the two income tests are not the same test.
If you are coming off employer or retiree coverage
COBRA is not a substitute for enrolling on time, and losing employer coverage that paid after Medicare is itself one of the federal guaranteed-issue triggers — with a 63-day deadline attached.
When Medicare says no, or costs more than expected
- Medicare billing appeals — the five levels
- Medicare Advantage prior authorization — what the OIG found
- How Medicare brokers get paid — the cap that only covers half
The one thing worth deciding slowly
Almost everything at 65 can be changed later. The coverage choice is the exception, and it is the one most often made on premium alone in the week the paperwork is due. If nothing else from this page survives, let it be this: find out what your state lets you do in year five before you decide what to do in year one. The 51-state table.
General information drawn from Medicare, CMS and state insurance-department publications, not insurance advice. Medicare rules, plan availability and dollar figures change every year, and state rules differ. Every figure here is year-labelled and every source is named so you can check it against your own situation. We do not sell insurance and receive nothing if you buy a plan.
Lost in the terminology? The Medicare glossary defines the words that quietly cost money — benefit period, observation status, guaranteed issue, underwriting — each one separately linkable.
Later, when you want to change something. Which window allows which change is not obvious and the January one is narrower than it sounds: AEP versus MA open enrollment.