This question is answered by Medicare.gov: https://www.medicare.gov/sign-up-change-plans/how-do-i-get-parts-a-b/part-a-part-b-sign-up-periods
Once your Initial Enrollment Period ends, you may have the chance to sign up for Medicare during a Special Enrollment Period (SEP). If you’re covered under a group health plan based on current employment, you have a SEP to sign up for Part A and/or Part B anytime as long as:
- You or your spouse (or family member if you’re disabled) is working.
- You’re covered by a group health plan through the employer or union based on that work.
You also have an 8-month SEP to sign up for Part A and/or Part B that starts at one of these times (whichever happens first):
- The month after the employment ends
- The month after group health plan insurance based on current employment ends
Usually, you don’t pay a late enrollment penalty if you sign up during a SEP.