Medicare enrollment has more timing rules than most people expect, and getting them wrong can cost you real money in the form of permanent premium penalties or gaps in coverage. The good news is that once you understand the main windows, it becomes pretty straightforward.

Initial Enrollment Period (IEP): Your First Chance

When you turn 65, you get a seven-month window to sign up for Medicare. This window is called the Initial Enrollment Period, and it runs three months before your birthday month, through your birthday month, and three months after your birthday month.

For example, if your birthday is in June, your Initial Enrollment Period runs from March through September.

When your coverage starts depends on when you sign up within that window.

The practical advice: sign up a few months before you turn 65 if you want seamless coverage from day one.

If You Have Employer Coverage at 65

This is where a lot of people get tripped up. If you are still working at 65 and covered by an employer health plan through either your own job or a spouse's, you have some choices -- and some timing considerations that matter a lot.

If you work for a company with 20 or more employees, your employer plan is the "primary" payer and Medicare is secondary. You can often delay Medicare Part B enrollment without penalty as long as you have that employer coverage. You get a Special Enrollment Period to sign up for Part B within eight months of losing that coverage (either because you retire or lose the employer plan).

If you work for a company with fewer than 20 employees, Medicare becomes your primary insurer at 65 even if you have employer coverage. In this case, you really do need to sign up for Medicare on time or risk coverage gaps and penalties.

Important: COBRA coverage and retiree health coverage are NOT considered employer coverage for Medicare purposes. If you are on COBRA after leaving a job, you still need to sign up for Medicare during your Initial Enrollment Period or face late penalties.

General Enrollment Period: If You Missed Your Window

If you did not sign up for Medicare when you first became eligible and you do not qualify for a Special Enrollment Period, you can sign up during the General Enrollment Period, which runs January 1 through March 31 each year. Coverage then starts on July 1.

The downside is the late enrollment penalty. For Part B, the penalty is a 10% surcharge on your premium for each full 12-month period you went without coverage. This penalty is permanent -- it stays with you for as long as you have Medicare.

Annual Enrollment Period (AEP): Switching Plans Each Fall

The Annual Enrollment Period runs October 15 through December 7 every year. This is when anyone with Medicare can make changes for the following year.

During this period you can switch from Original Medicare to a Medicare Advantage plan, switch from Medicare Advantage back to Original Medicare, switch from one Medicare Advantage plan to another, or change your Part D drug plan. Changes take effect January 1 of the following year.

This is also when you should be reviewing your Part D plan every year, because formularies, premiums, and pharmacy networks change annually.

Medicare Advantage Open Enrollment: January Through March

If you enrolled in a Medicare Advantage plan during the fall and have second thoughts after January 1, you have one more window. The Medicare Advantage Open Enrollment Period runs January 1 through March 31 and allows you to switch to a different Medicare Advantage plan or switch back to Original Medicare. You can only make one change during this period.

Special Enrollment Periods

Outside of the standard enrollment windows, certain life events trigger Special Enrollment Periods. These include losing employer coverage, moving to a new area, losing Medicaid eligibility, leaving a facility like a nursing home, and others.

If you think you qualify for a Special Enrollment Period, contact Social Security (1-800-772-1213) or your State Health Insurance Assistance Program for guidance. Acting quickly matters -- most Special Enrollment Periods have limited windows.

Bottom Line

For most people turning 65, sign up a few months before your birthday to avoid any coverage gaps. If you are still working with employer insurance, understand whether your company size affects your rules before you delay enrollment.

Disclaimer: The information on this site is for educational purposes only and does not constitute legal, financial, or medical advice. Medicare rules and costs change annually. Always verify current information at Medicare.gov or by calling 1-800-MEDICARE. Consider consulting a licensed insurance professional or your State Health Insurance Assistance Program (SHIP) for personalized guidance.