Medicare enrollment. Social Security beneficiaries are often automatically enrolled in Medicare Parts A and B, with coverage starting the month they turn 65. (If your birthday falls on the first of the month, coverage will start at the beginning of the prior month.) Medicare cards are mailed out to most Social Security beneficiaries three months before their 65th birthday, and Medicare Part B premiums are withheld from Social Security payments. However, you will need to actively sign up if you want prescription drug coverage, a Medicare supplemental policy or a Medicare Advantage plan. Workers who haven't signed up for Social Security will need to enroll in Medicare and pay the premiums out of pocket in order to receive coverage. You can sign up online at socialsecurity.gov, by phone at 1-800-772-1213, or at your local Social Security office.