Supplement plans require enrollment into a Medicare plan. Individuals who are approaching Medicare age have a full seven-month window in which to enroll in Medicare, which includes the three months before your 65th birthday, the month OF your 65th birthday, and the three months after your 65th birthday. If you sign up in the three-month window before your birthday, coverage effective date will begin in the month of your 65th birthday. Enrolling after your birthday could create a delayed effective date.