Medicare enrollment windows (and the penalties that never expire)

Medicare is the rare government program that punishes lateness with a lifetime surcharge: enroll in Part B late and your premium goes up 10% for every year you dawdled — forever. Enroll in Part D late and a smaller penalty also sticks forever. The rules are genuinely confusing, especially for the growing number of people working past 65, and the most expensive mistakes are made by people who thought they were doing everything right.

The windows that matter

• Initial Enrollment Period (IEP): 7 months around your 65th birthday — the 3 months before your birthday month, your birthday month, and the 3 months after. Enroll before your birthday month and coverage starts the month you turn 65.
• General Enrollment Period: January 1 – March 31 each year, for people who missed their IEP — with penalties attached.
• Special Enrollment Period (SEP): 8 months after employer coverage ends (for Part B), if you delayed because you had job-based insurance from active employment. Penalty-free.
• Medicare open enrollment (October 15 – December 7): for changing Advantage and Part D plans, not for first-time Part B enrollment.

The penalties, precisely

Part B: 10% added to your premium for each full 12-month period you were eligible but not enrolled — permanently. Part D: 1% of the national base premium for each month you lacked drug coverage (or other 'creditable' coverage) after your IEP — also permanent. Part A is premium-free for most people and worth taking at 65 unless you're contributing to an HSA (more on that below).

Still working at 65? Read this twice

• Employer with 20+ employees: your group plan pays primary and you can delay Part B penalty-free. You'll get an 8-month SEP when employment or the coverage ends.
• Employer with fewer than 20 employees: Medicare pays primary at 65 even if you're still on the group plan — skip Part B and you can be left with almost no real coverage. You generally must enroll.
• COBRA and retiree coverage do NOT count as active employment coverage. Going on COBRA at 64½ and 'delaying' Medicare is one of the classic penalty traps — your Part B clock runs anyway.
• HSA contributors: enrolling in any part of Medicare (including premium-free Part A) ends HSA eligibility. And if you claim Social Security past 65, Part A is backdated up to 6 months — stop HSA contributions about 6 months before you enroll or file, or face excise taxes.

Don't forget Part D — even if you take no medications

The Part D penalty clock runs whenever you lack creditable drug coverage, healthy or not. If you're not on employer coverage, enrolling in a cheap Part D plan — sometimes around $10–$15/month — at 65 is penalty insurance: it caps your future costs and keeps you covered when prescriptions inevitably arrive. Each September, employer and retiree plans must tell you whether their drug coverage is 'creditable'; keep that letter.

Your pre-65 checklist

1. At 64: mark your 7-month IEP on the calendar and decide whether you'll still have large-employer coverage at 65.
2. Confirm your employer's size category and whether its drug coverage is creditable — in writing.
3. If HSA-contributing, plan your contribution stop date around Part A enrollment or Social Security filing.
4. Enroll during the 3 months before your birthday month for seamless coverage, at ssa.gov.
5. Losing employer coverage later? Start your 8-month SEP paperwork immediately — and remember the Medigap guaranteed-issue window is shorter (6 months from Part B enrollment).

The bottom line

Medicare's deadlines are unforgiving and its penalties are permanent, but the rules are learnable: enroll in your 7-month window at 65 unless you have coverage from a 20+ employee employer, never treat COBRA as a reason to delay, mind the HSA interaction, and buy cheap Part D as penalty insurance. One afternoon of planning at 64 protects every retirement year that follows.

The windows and penalties on one card

Notice the last row, because it is the deadline people discover too late: the Medigap guaranteed-issue window is shorter than everything else and does not reopen. Enrolling in Part B starts a six-month clock during which no supplement insurer can deny you or price you on health. Someone who takes Part B at 65, tries a Medicare Advantage plan for a few years, and then develops a heart condition at 70 may find Medigap either unaffordable or unavailable in most states. The B-then-Medigap sequencing decision deserves as much attention as the enrollment dates themselves — the penalty for getting it wrong is not a surcharge but a locked door.

A brief word on how the two lifetime penalties actually compound, since 'for life' can feel abstract. The Part B penalty is calculated on the standard premium, which rises most years — so a 30% penalty grows in dollar terms automatically, from roughly $55 a month today to perhaps $90 a month a decade from now. The Part D penalty is recalculated annually against the national base premium, so it creeps too. A couple who both enroll three years late are staring at something like $1,300–1,600 a year of pure penalty by their late seventies — money that buys nothing, forever, because of a deadline that one calendar reminder at 64 would have caught. Set the reminder. It is the single highest-yield minute of retirement administration that exists.