For most people who become eligible at 65, the first chance to enroll in Medicare Part A and Part B is a seven-month Initial Enrollment Period. It starts three months before the month you turn 65, includes your birthday month, and ends three months after it.

The short versionDo not treat your 65th birthday as the deadline. Treat the three months before your birthday month as your planning window, especially if you want coverage ready when other insurance ends.

Your seven-month window

  1. Three months before your birthday monthYour Initial Enrollment Period opens. This is the safest time to confirm whether enrollment is automatic and what other coverage you expect to keep.
  2. Your birthday monthYou are in the middle of the Initial Enrollment Period. Coverage always starts on the first of a month, and the exact start date depends on when you enroll.
  3. Three months after your birthday monthYour Initial Enrollment Period remains open, but waiting can change the coverage start date and create a gap.

Medicare.gov has an important special rule for people whose birthday falls on the first day of a month. Use Medicare’s official start-date tool rather than assuming the standard birthday-month timeline applies exactly the same way.

Enrollment may be automatic — or it may require action

Some people receive Part A and Part B automatically, while others must apply through Social Security. Automatic enrollment often depends on whether you are already receiving Social Security or Railroad Retirement Board benefits.

Check your own status through Medicare.gov’s official enrollment guide. If you need to apply, Medicare directs applicants to Social Security’s Medicare application.

Working past 65? Do not use a one-size-fits-all answer

Delaying Part B can make sense for some people covered by an employer group health plan based on current employment. But the right answer can depend on who is working, the kind of coverage, employer size, and when employment or coverage ends.

COBRA is not treated as current-employment group health coverage for this decision. Medicare says the eight-month Special Enrollment Period for Part B starts when work or qualifying job-based coverage ends, even if you elect COBRA afterward.

Before delaying Part B, confirm these points with the employer benefits administrator and the official Medicare questionnaire:

  • Is the coverage based on your current employment or your spouse’s current employment?
  • How many employees does the employer have?
  • Which coverage pays first after age 65?
  • When exactly will employment and job-based coverage end?
  • Will you contribute to a Health Savings Account? Medicare enrollment can affect HSA eligibility and timing.

Medicare’s interactive “When can I sign up?” tool is the correct starting point because it changes the answer based on the coverage you actually have.

If you miss the first window

If no Special Enrollment Period applies, the General Enrollment Period runs from January 1 through March 31 each year. Medicare says coverage starts the month after you enroll. A late-enrollment penalty may apply, and the Part B penalty can continue for as long as you have Part B.

That is why the most useful question is not simply “Do I have insurance?” It is: “Do I have the kind of current-employment coverage that lets me delay Part B without a gap or penalty?”

A practical planning checklist

  1. Write down your seven-month Initial Enrollment Period.
  2. Check whether enrollment will be automatic.
  3. List every coverage source you expect to have at 65.
  4. If working, get written answers from the benefits administrator.
  5. Use Medicare.gov’s official enrollment questionnaire.
  6. Apply early enough for the coverage start date you need.
  7. Keep confirmation pages, letters, and benefit records in one place.

Official sources used

Sources last checked Aug. 28, 2026. Medicare rules and individual circumstances can change; confirm your dates directly with Medicare or Social Security before acting.