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Medicare does not start automatically for everyone, and the enrollment windows have hard edges. Sign up in the wrong order or at the wrong time and you can end up paying penalties for the rest of your life. The rules are not complicated once you see them laid out, so here they are.
Your initial enrollment period: seven months
Your initial enrollment period is the seven-month window around your 65th birthday: the three months before the month you turn 65, your birthday month, and the three months after. If your birthday falls on the first of the month, the whole window shifts one month earlier.
If you are already receiving Social Security or Railroad Retirement benefits, enrollment in Parts A and B is automatic. Everyone else has to sign up, online, by phone, or in person at a Social Security office. When you enroll in the three months before your birthday month, coverage starts on the first day of your birthday month. Enroll later in the window and coverage starts later, which can leave a gap.
The Part B late penalty: 10 percent per year, forever
If you do not sign up for Part B when you are first eligible and you do not have qualifying coverage from current employment, you pay a late enrollment penalty of 10 percent of the Part B premium for each full 12-month period you delayed. It is added to your premium for as long as you have Part B. A two-year delay means a 20 percent surcharge every month, permanently.
The key exception: if you are still working and covered by a group health plan through your or your spouse’s current employer, you generally get a special enrollment period when that employment or coverage ends, and no penalty. But “current employer” matters. COBRA and retiree coverage do not count.
The Part D late penalty: 1 percent per month, forever
Drug coverage has its own penalty. If you go 63 or more days without creditable prescription coverage after your initial enrollment period, you pay 1 percent of the national base beneficiary premium for each month you went without it, for as long as you have Part D. Creditable means coverage at least as good as standard Part D, which most employer plans are, but you should confirm in writing.
If you missed the window
There is a general enrollment period each year from January 1 through March 31 for Part A and Part B, with coverage starting the month after you enroll. It exists as a backstop, not a strategy. You will still owe the late penalties, and you will have spent months uninsured.
Special enrollment periods cover the common life changes: losing employer coverage, moving out of a plan’s service area, losing Medicaid eligibility. These give you a limited window to sign up without penalty, usually measured in months, not years. If any of these apply to you, enroll as soon as the triggering event happens rather than waiting to see how the deadlines work.
A simple timeline
Three months before you turn 65, decide whether you need Parts A and B now or whether employer coverage lets you delay. Sign up for Part D at the same time, even if you take no medications, to avoid the penalty. If you are retiring before 65, read our guide to bridging the gap to Medicare so you do not end up uninsured in the years before eligibility. And if you have not yet sorted out what each part actually does, start with the parts A through D explainer.
