Medicare enrollment timeline — the dates that matter

Reviewed by the How To Help Your Elders Team

Medicare has hard deadlines, and missing them costs real money, sometimes permanently. According to CMS, approximately 750,000 Medicare beneficiaries pay late enrollment penalties on their Part B premiums every year, penalties that never go away. This guide walks through every enrollment window, explains what happens if deadlines slip, and gives you the dates to mark on your calendar so your parent's coverage stays intact and penalty-free.

Initial Enrollment Is a Seven-Month Window Around Age 65

Your parent becomes eligible for Medicare at age 65. The Initial Enrollment Period (IEP) is a seven-month window: three months before the month your parent turns 65, the birthday month itself, and three months after. If your parent is already receiving Social Security benefits, they will be automatically enrolled in Medicare Part A (hospital insurance) and Part B (medical insurance) effective the first day of their birthday month. If your parent is not yet receiving Social Security, they need to actively enroll during this window.

Part D (prescription drug coverage) has a separate enrollment decision. Even if your parent currently takes no medications, CMS strongly recommends enrolling in Part D during the IEP. Declining Part D and enrolling later triggers a late enrollment penalty of 1 percent of the national base beneficiary premium for every month they were eligible but not enrolled. That penalty is added to their monthly premium and never goes away. For someone who delays Part D enrollment by five years, the penalty adds roughly $20 per month to their premium for life.

If your parent is still working at 65 and has employer health insurance, delaying Medicare enrollment is allowed without penalty, provided the employer plan is based on current employment (not COBRA or retiree coverage). When your parent leaves that job or loses that employer coverage, they have an eight-month Special Enrollment Period to sign up for Medicare without penalty. The timing of that transition matters. Your parent should coordinate with their employer's benefits administrator and with Social Security to make sure there is no gap in coverage.

Open Enrollment Runs October 15 Through December 7 Every Year

The Annual Open Enrollment Period (AEP) is the window when your parent can change their Medicare coverage. It runs from October 15 through December 7, with changes taking effect January 1. During this period, your parent can switch from Original Medicare to a Medicare Advantage plan (Part C), switch between Medicare Advantage plans, return to Original Medicare from an Advantage plan, or change their Part D prescription drug plan.

CMS reports that only about 10 percent of Medicare beneficiaries actively review and compare plans during Open Enrollment, even though plan costs and coverage change every year. A plan that was the best value last year may have increased its premiums, raised its deductible, or dropped your parent's doctor from the network. Spending an hour on Medicare.gov's plan comparison tool during October can save hundreds or thousands of dollars over the following year.

For Medicare Advantage enrollees, there is also a Medicare Advantage Open Enrollment Period from January 1 through March 31. During this narrower window, someone on a Medicare Advantage plan can switch to a different Advantage plan or return to Original Medicare and enroll in a standalone Part D plan. Changes made during this period take effect the first of the month after the plan receives the enrollment request.

Special Enrollment Periods Cover Life Changes

Life events outside the regular enrollment windows can trigger a Special Enrollment Period (SEP). Qualifying events include losing employer health coverage, moving to a new state, gaining or losing Medicaid eligibility, and certain other changes in circumstances. The SEP is typically 60 days from the qualifying event.

The most common SEP situation is retirement. When your parent retires and loses employer coverage, the eight-month SEP starts the month after employment ends or the month after employer coverage ends, whichever comes first. During those eight months, your parent can enroll in Part A, Part B, and Part D without penalty. Missing this window means waiting for the General Enrollment Period (January 1 through March 31), during which coverage does not start until July 1, creating a gap that can last months.

Moving to a new state qualifies for a SEP because the move may put your parent outside their current plan's service area. If their doctors are no longer in-network or their plan does not operate in the new state, they need to switch. CMS requires documentation of the move, so keeping records of the address change helps the process go smoothly.

Missing Deadlines Has Permanent Consequences

The Part B late enrollment penalty is a 10 percent increase to the monthly premium for every full 12-month period your parent was eligible but not enrolled. This penalty is permanent. If your parent delays Part B enrollment by three years without qualifying employer coverage, they pay 30 percent more on their Part B premium for the rest of their life. In 2025, the standard Part B premium is $185 per month. A 30 percent penalty adds $55.50 per month, or $666 per year, permanently.

The Part D late enrollment penalty works similarly. For every month your parent goes without creditable prescription drug coverage after their Initial Enrollment Period, 1 percent of the national base beneficiary premium is added to their monthly Part D premium. This penalty also never expires.

If your parent misses the Annual Open Enrollment Period and wants to change plans, they are locked into their current plan until the next fall unless a Special Enrollment Period applies. Being stuck in a plan that does not work well for an entire year is frustrating, costly, and avoidable by marking the October 15 through December 7 window on the calendar every year.

The Dates to Put on Your Calendar

Your parent's 65th birthday minus three months is when the Initial Enrollment Period opens. That date matters more than the birthday itself because enrolling in the first three months of the IEP means coverage starts on the first day of the birthday month.

October 15 through December 7 is Open Enrollment, every year, no exceptions. Review plans during this window and make any changes before December 7 for January 1 coverage.

If your parent retires or loses employer coverage, the eight-month Special Enrollment Period clock starts immediately. Do not wait. Enroll in Medicare within the first few weeks to avoid any gap in coverage.

January 1 through March 31 is the Medicare Advantage Open Enrollment Period for anyone already on an Advantage plan, and also the General Enrollment Period for anyone who missed their initial window. Coverage from a General Enrollment Period enrollment does not start until July 1.

These dates are not flexible. CMS does not grant extensions for forgetting or not knowing. Putting reminders on a calendar, setting phone alerts, or asking your parent's insurance broker to flag these dates prevents the kind of mistake that follows your parent's finances for the rest of their life.

Frequently Asked Questions

What happens if my parent is automatically enrolled in Medicare but doesn't want Part B?
Your parent can decline Part B, but doing so means they will not have coverage for doctor visits, outpatient services, or preventive care. If they change their mind later, they can only enroll during the General Enrollment Period (January through March), coverage starts in July, and they will pay the late enrollment penalty permanently.

Does my parent need Part D if they don't take any medications?
CMS recommends enrolling in Part D even if your parent currently takes no prescriptions. The late enrollment penalty for delaying Part D adds up over time and never goes away. A low-premium Part D plan provides coverage in case medications become necessary later without incurring penalties.

Can my parent keep their employer insurance and Medicare at the same time?
Yes. If your parent is still working and has employer coverage, they can delay Medicare enrollment without penalty. Once they stop working or lose employer coverage, they must enroll in Medicare within eight months. Coordination between employer coverage and Medicare should be discussed with the employer's benefits department.

What if my parent moves to a different state?
Moving triggers a Special Enrollment Period. Your parent has 60 days to change their Medicare Advantage plan or Part D plan to one that operates in the new state and includes local providers. Keep documentation of the move, such as a new lease or utility bill, as CMS may request proof.

How do I compare Medicare plans during Open Enrollment?
Medicare.gov has a plan finder tool where you enter your parent's zip code, current medications, and preferred doctors to see side-by-side comparisons of available plans. The State Health Insurance Assistance Program (SHIP) in your state also offers free, unbiased counseling to help compare options. Call 1-800-MEDICARE to find your local SHIP office.