Turning 65 with a job is more complicated than turning 65 without one, and the complications are almost entirely paperwork. Get the paperwork right and this is a non-event. Get it wrong and you can carry a premium penalty for the rest of your life.
Your Initial Enrolment Period is seven months
Three months before the month you turn 65, your birthday month, and the three months after. If you are already taking Social Security you are generally enrolled in Part A and Part B automatically. If you are not, nothing happens unless you act.
That second sentence is the one we repeat most often. There is no letter, no phone call and no default. Plenty of people discover it in month eight.
Employer size changes the rules
If your employer has 20 or more employees, the group health plan is generally primary and Medicare is secondary. In that situation many people delay Part B without penalty and take a Special Enrolment Period when the employment or the coverage ends.
If your employer is smaller than that, Medicare can be primary, and staying off Part B can leave you effectively uninsured for the share Medicare would have paid. This is the single most consequential question in the conversation, and it is answered by counting employees, not by asking how good the plan feels.
Part A is usually free, but not always harmless
Most people qualify for premium-free Part A and take it at 65 even while working. There is one important exception: if you contribute to a Health Savings Account, enrolling in any part of Medicare ends your eligibility to contribute.
Worse, Part A can be backdated up to six months when you enrol late, which can retroactively disqualify contributions you already made. If you have an HSA, stop contributions in good time before your Medicare start date and talk to your tax adviser about the months in question.
Keep the creditable coverage letter
Ask your HR department for a letter confirming that the employer plan provided creditable coverage, including drug coverage, and the dates. File it somewhere you will find it in four years.
That letter is what prevents a Part B or Part D late enrolment penalty argument when you eventually enrol. Nobody ever regrets having it and plenty of people spend weeks trying to recreate it from a former employer that has since been acquired.
The Medigap window you only get once
When you do take Part B, a six-month Medigap open enrolment window opens. During it, carriers must issue you a Medicare Supplement policy regardless of your health. After it closes, most Tennessee applications go through medical underwriting and can be declined.
This matters even if you are leaning toward Medicare Advantage, because switching from Advantage to a Supplement later is where the underwriting question comes back. Knowing that in advance is not a reason to choose one over the other. It is a reason to choose deliberately.
A sensible sequence
- At 64 and a half: count your employer's employees and request the creditable coverage letter
- Three months before your birthday month: decide on Part B, and stop HSA contributions if Medicare is starting
- At Part B start: decide Advantage or Supplement plus Part D, with your drug list and doctors in hand
- Every October: re-run the drug list against next year's plans before December 7
An estimate for planning, not a quote. HealthCare.gov sets your real credit.
