Published 2026-09-12 · Last updated 2026-09-12.
Initial Enrollment Period — your first window
Your Initial Enrollment Period is the seven-month window around the month you turn 65: the three months before it, your birthday month, and the three months after it. During this window you can sign up for Parts A and B and join an Advantage or drug plan. If you are already receiving Social Security benefits when you turn 65, enrollment in Parts A and B is generally automatic and your card arrives by mail.
If you are not yet receiving benefits, you must sign up yourself, and the month you sign up within the window affects when cover starts. Do not leave this to the last month by default — an early application gives you room to fix a document problem without losing your intended start.
Part B enrollment — the decision most people delay
Part B enrollment deserves its own attention because delaying it without other qualifying cover can lead to a late-enrollment penalty that lasts as long as you hold the cover. The penalty grows with the length of delay, in general terms, which is why 'I will sort it later' is the riskiest sentence in Medicare planning. If you have qualifying employer cover past 65, the rules differ — but verify that your employer cover actually qualifies, since the test depends on employer size and cover type.
Sign up through Social Security — online, by phone or in person — and keep the confirmation. If you are still working, ask both Social Security and your employer plan administrator, in writing, whether your current cover counts as qualifying, and keep their answers.
Part D enrollment — drug cover on the same clock
Part D follows the same initial window: join a standalone drug plan alongside Original Medicare, or get drug cover bundled inside an Advantage plan. Going without drug cover or other creditable drug cover after first eligibility can also lead to a late penalty, calculated in general terms on the national base premium and lasting as long as you hold the cover.
Creditable means the cover is at least as good as standard Part D, on average. If an employer or other plan claims to be creditable, get that statement in writing and keep it — it is your protection if the question ever arises later. Check your medicines against any drug plan's formulary before you join, not after.
Penalties in general — and Special Enrollment
Penalties are best understood as a reason to act on time, not as a punishment to fear. The general shape is consistent: delay without qualifying cover adds a lasting surcharge to the relevant premium. Special Enrollment Periods exist for qualifying events — losing employer cover, moving, and similar changes — and they let you enrol outside the main windows without penalty, provided you act within the period the event opens.
If you missed your initial window, the General Enrollment Period is the annual catch-up window for Parts A and B, with cover generally starting the month after you sign up. Do not wait for it if a special period is available to you now; earlier cover is almost always better than later cover.
Switching windows — after you are in
Once you are in Medicare, the annual open enrollment window lets you switch between Original Medicare and Advantage, change plans, or join, switch or drop a drug plan, with changes starting the following January. Advantage members also get a separate early-year window to make one change. Mark both windows and review your plan each year — doctors, medicines and premiums all change, and last year's best plan is not automatically this year's.
Before any switch, re-run the cost calculator and the plan comparison with your current doctors and medicines. A switch that looks good on premium alone, without a network and formulary check, is the most common Medicare regret after a late-enrollment penalty.
ILLUSTRATIVE EXAMPLE
ILLUSTRATIVE EXAMPLE (not a policy fact): Frank, a fictional worker turning 65 while still employed, asks his employer plan administrator and Social Security — in the example — whether his employer cover qualifies, keeps both answers in writing, and enrols in Part B during a special window when that cover ends. He joins a drug plan in the same move so no drug-cover gap opens. No date, penalty figure or employer-size threshold in this example is asserted as a fact.