Most people sign up for both Part A and Part B at the same time when they are first eligible as soon as they turn 65. However, in some situations it might make sense to wait and sign up for Medicare Part B later. There are risks to delaying your enrollment and signing up later such as a gap in coverage or having to pay a late enrollment penalty. If you are already receiving Social Security benefits, you will automatically be enrolled in Medicare Part B. Your card will arrive in the mail about 3 months before your birth month. If you do not receive Social Security benefits, you will have to enroll yourself. You can do so online, on the phone or in-person at your local Social Security Office.