You are within a year or two of 65, or you are past it and still working. The most expensive Medicare mistakes are not about picking the wrong plan — they are about missing a deadline, and the penalty for missing it is added to your premium for the rest of your life.
The Initial Enrollment Period: 7 months
Medicare eligibility arrives at 65, and your main on-ramp is the Initial Enrollment Period: seven months built around your 65th birthday month.
| When you sign up | When coverage starts |
|---|---|
| The 3 months before your birthday month | The first day of your birthday month |
| Your birthday month | The first day of the next month |
| The 3 months after your birthday month | The first day of the month after you sign up |
Sign up in the three months before your birthday month and coverage starts at 65 with no gap. Part A is premium-free if you (or a spouse) paid Medicare tax for 10 years, so the deadlines that bite are about Part B (doctors and outpatient care, $202.90 a month in 2026) and Part D (drugs) — the costs lesson has the full price list.
The Special Enrollment Period: still working at 65
If you are covered by a group plan from your or your spouse's current job, and the employer has 20 or more employees, you can delay Part B without penalty. When the job or the coverage ends, you get an 8-month Special Enrollment Period to sign up, and coverage starts the first day of the month after you apply. You apply through ssa.gov or a Social Security office and supply proof of the employer coverage and its dates. Sign up for Part B before the job ends and there is no gap at all.
| Your situation at 65 | Your path |
|---|---|
| No coverage from a current job | Enroll in the 7-month Initial Enrollment Period |
| Still working, employer plan, 20+ employees | Delay Part B; enroll during the 8-month Special Enrollment Period when the coverage ends |
| Employer has fewer than 20 employees | Enroll at 65 — Medicare pays first and the small-employer plan pays second |
| Missed both windows | The General Enrollment Period, January 1 to March 31 each year, with coverage starting the month after you sign up — and the penalty below |
Which coverage lets you delay — and which does not
| Coverage at 65 | Penalty-free delay? |
|---|---|
| Your own or your spouse's current employer plan (20+ employees) | Yes |
| COBRA continuation coverage | No |
| Retiree health coverage from a former employer | No |
| A Marketplace (ACA) plan | No |
| No coverage | No — and you have a gap as well as a penalty |
Only coverage from current, active employment opens a Special Enrollment Period. COBRA is the trap: it feels like employer coverage, and it is not. Medicare also counts the 8-month window from the day the job ends, not the day COBRA ends.
The penalties that never go away
The late-enrollment penalties for Part B and Part D are not one-time fees. They are surcharges added to your monthly premium for as long as you have that coverage — for life — and they grow with every month of delay.
| Penalty | How it is calculated | How long you pay it |
|---|---|---|
| Part B | 10% of the standard premium for each full 12-month period you could have had Part B and did not | For as long as you have Part B |
| Part D | 1% of the national base beneficiary premium ($38.99 for 2026) for each month you had no creditable drug coverage, rounded to the nearest $0.10 | For as long as you have Medicare drug coverage |
The Part D penalty starts after 63 days in a row without Part D or other creditable coverage (a plan at least as good as Medicare's — your employer plan sends a notice each year saying whether it counts). Because the Part D penalty is recomputed each year on that year's base premium, it rises with inflation too.
With the deadlines settled, the next lesson turns to the structural choice: Original Medicare with a supplement, or Medicare Advantage.