Medicare has three main enrollment windows each year, and missing them can cost you thousands in penalties

Initial Enrollment Period runs for seven months centered on your 65th birthday — three months before, the month you turn 65, and three months after. This is your one chance to join without a late penalty. If you miss it, you pay a permanent surcharge on your premiums for as long as you have Medicare.

General Enrollment Period runs January 1 through March 31 every year. You can join or switch plans during this window, but again, missing your Initial Enrollment Period means a penalty that never goes away.

Annual Enrollment Period (also called Open Enrollment) runs October 15 through December 7 each year. This is when people already on Medicare can change plans — switch from Original Medicare to a Medicare Advantage plan, move between Advantage plans, or change prescription drug coverage. Changes take effect January 1.

There are also Special Enrollment Periods if you have a may have access to life event — you lose employer coverage, move out of your plan's service area, or experience certain other changes. These let you enroll outside the normal windows without penalty.

Key Takeaways

  • Your Initial Enrollment Period is seven months long: three months before your 65th birthday, the month you turn 65, and three months after — and you can only use it once.
  • If you miss your Initial Enrollment Period, you pay a permanent 10% surcharge on Part B premiums and a 1% surcharge on Part D premiums for every 12 months you delay.
  • General Enrollment Period (January 1 to March 31) lets you join if you missed your initial window, but the penalty still applies.
  • Annual Enrollment Period (October 15 to December 7) is when you can switch plans if you are already on Medicare, with changes taking effect January 1.
  • Special Enrollment Periods open outside normal windows if you lose coverage, move, or experience certain life changes — you must act within 60 days of the event.

Your Initial Enrollment Period: The seven-month window you cannot get back

Medicare counts your Initial Enrollment Period from the first day of the month you turn 65, then adds three months before and three months after. So if you turn 65 on March 15, your window opens December 1 and closes June 30. The exact dates depend on your birthday, but the length is always seven months.

You must enroll in Part A (hospital insurance) and Part B (medical insurance) during this period if you want coverage to start on your 65th birthday month. If you enroll in the first three months (before your birthday month), coverage starts the month you turn 65. If you enroll in your birthday month or the three months after, there is a one-month delay for each month you wait.

This window is one-time only. You cannot extend it, and you cannot use it again. If you turn 65 on March 15, 2024, your Initial Enrollment Period closes June 30, 2024. After that, the only way to join is through General Enrollment Period (January through March each year), and you will owe a late penalty for every month you were not enrolled.

Late enrollment penalties: The permanent cost of missing your important date

If you do not enroll in Part B during your Initial Enrollment Period, Medicare charges you a 10% surcharge on your Part B premium for every 12 months you delay. That surcharge is permanent — it stays on your bill for the rest of your life, even after you finally join.

The same rule applies to Part D (prescription drug coverage). If you go 63 days or more without creditable drug coverage and then join, you pay a 1% surcharge on your Part D premium for every month you were uninsured. That penalty also lasts for life.

Example: You turn 65 on March 15, 2024, but do not enroll. You wait until February 2025 and join through General Enrollment. You were uninsured for 11 months. Medicare adds a 10% surcharge to your Part B premium and a 11% surcharge to your Part D premium, and both stay on your bill permanently.

The only exception is if you had creditable coverage through an employer or union while you were 65 or older. If you can prove that coverage, you may not owe a penalty. You must show documentation from your employer or former employer stating the coverage was creditable.

General Enrollment Period: Your backup window if you missed the first one

General Enrollment Period runs January 1 through March 31 every year. If you did not enroll during your Initial Enrollment Period, you can join Part A and Part B during this window. Coverage starts July 1 of that year.

The catch is that the late penalty still applies. If you missed your Initial Enrollment Period by any amount of time, you owe the surcharge. General Enrollment Period is a second chance to join, not a second chance to avoid the penalty.

You can also use General Enrollment to switch between Original Medicare and Medicare Advantage, or to add or change Part D coverage. However, if you are already enrolled, the Annual Enrollment Period (October 15 to December 7) is usually the better time to make changes, because it has fewer restrictions.

Annual Enrollment Period: When to change plans if you are already on Medicare

Annual Enrollment Period runs October 15 through December 7 each year. If you are already on Medicare, this is when you can make changes to your coverage. You can switch from Original Medicare to a Medicare Advantage plan, move from one Advantage plan to another, drop or add Part D coverage, or switch between Part D plans.

Changes you make during Annual Enrollment take effect January 1 of the following year. If you enroll in a new plan on November 1, your coverage switches on January 1. If you wait until December 7 (the last day), your coverage still switches on January 1.

You do not need a reason to make changes during Annual Enrollment. You can switch plans straightforward because you want to. However, if you want to make changes outside this window, you need a may have access to life event — and those changes must be made within 60 days of the event.

Special Enrollment Periods: When you can enroll outside the normal windows

A Special Enrollment Period opens if you experience certain life changes. The most common triggers are losing employer coverage, moving out of your plan's service area, or having your plan leave the Medicare program. You have 60 days from the date of the event to make changes.

Other may have access to events include getting married, getting divorced, becoming a U.S. citizen, or having a change in your Medicaid status. If you are unsure whether your situation qualifies, contact Medicare at 1-800-MEDICARE (1-800-633-4227) and describe what happened. They can tell you whether you have a Special Enrollment Period and how long you have to act.

The 60-day clock starts on the date of the event, not the date you find out about it. If your plan leaves Medicare on June 1 but you do not realize it until July 15, your 60 days started on June 1. You have until August 30 to enroll in a new plan. Missing that important date means you cannot change until the next Annual Enrollment Period.

How to enroll: Where to go and what to have ready

You can enroll in Medicare three ways: online at Medicare.gov, by phone at 1-800-MEDICARE (1-800-633-4227), or in person at your local Social Security office.

Before you contact Medicare, have your Social Security number, birth date, and citizenship or immigration status ready. If you are enrolling in Part D or a Medicare Advantage plan, have a list of your current medications and doctors, because the plan details matter for your choice.

If you are already receiving Social Security benefits, Medicare may enroll you automatically in Part A and Part B. Check your Social Security statement or call Social Security at 1-800-772-1213 to confirm. Automatic enrollment happens about three months before your 65th birthday.

Enrollment by phone usually takes 10 to 15 minutes. Online enrollment through Medicare.gov takes longer but lets you review plan details side by side. In-person enrollment at Social Security is free and can be helpful if you have complex coverage questions, but wait times vary by location.

Frequently Asked Questions

What happens if I turn 65 while still working and have employer coverage?

You can delay enrolling in Medicare Part B without penalty as long as you have creditable coverage through your employer and are still actively working. When you leave your job or lose coverage, you have eight months to enroll in Part B without a late penalty. You must enroll in Part A at 65 unless you are still working and your employer has fewer than 20 employees.

Can I enroll in Medicare before I turn 65?

You can contact Medicare and start the enrollment process up to three months before your 65th birthday, but your coverage will not start until your 65th birthday month. Enrollment during the three months before your birthday is part of your Initial Enrollment Period, so starting early does not give you extra time — it just spreads the paperwork across more months.

What if I miss the important date for Annual Enrollment Period?

If you miss the October 15 to December 7 window and do not have a may have access to life event, you cannot change plans until the next Annual Enrollment Period. You are locked into your current plan for the rest of the year. The only exception is if you have a Special Enrollment Period due to a may have access to event like moving or losing coverage.

Do I have to enroll in Part D during my Initial Enrollment Period?

You do not have to, but if you go without creditable drug coverage for 63 days or more, you will owe a permanent 1% surcharge on Part D premiums for every month you were uninsured. If you do not take prescription drugs now, you can wait, but you should enroll before the 63-day gap to avoid the penalty.

Can I change my mind after I enroll?

Yes, but only during certain windows. If you enroll during your Initial Enrollment Period, you have until the end of that period to make changes. If you enroll during Annual Enrollment Period, you have until December 31 to change your mind. After that, you are locked in until the next enrollment period unless you have a may have access to life event.