When you turn 65 and sign up for Medicare Part B, you have a six-month open enrollment period for Medicare Supplements Plans. This period starts on the first day of the month of your 65th birthday. During open enrollment, you can purchase any Medicare Supplement policy without answering any medical questions. This is called guaranteed issue (GI). If you buy a Medicare Supplement outside your open enrollment, guarantee issue window, you may have to answer some medical questions. Depending on your answers, you may be declined or have to pay a higher premium. There can be exceptions, which is why it is so vital to work with a Medicare advisor to help you understand the options available in your unique situation.