This depends on a few factors, including how many employees your company has. For example: If your employer has 20 or more employees: You have the option of staying in your group plan or enrolling in Medicare. If you worked long enough to get Part A, keeping employer coverage at 65 may be more cost-effective. In this case, your employer coverage will be your primary insurance, with Part A acting as a secondary policy. You will be granted a Special Enrollment Period when you leave group coverage with an enrollment window of three months following your loss of group coverage. If your employer has fewer than 20 employees: Your employer may require you to enroll in Medicare when you turn 65 to serve as your primary insurance.