Medicare Supplement enrollment periods differ from other Medicare enrollment periods. Insurers must offer a six-month open enrollment period to all Medicare beneficiaries, regardless of health status, beginning with the first month in which you first enrolled for benefits under Part B. This open enrollment period begins on the first day of the month you were both 65 and older and enrolled in Part B. Once you are enrolled in a plan, it renews each year if you pay your premium, and the plan is available. After this six-month period ends, insurers may consider your health status for acceptance and premium.