Most Americans need to contact the Social Security Administration directly to enroll in Medicare when they become eligible at 65. Others, including those receiving Social Security who retired early and those receiving Social Security disability or Railroad Retirement benefits, are enrolled automatically when they become eligible at age 65.
Americans have a seven-month window, called the Initial Enrollment Period, around their 65th birthday, during which they can enroll in Medicare. Enrollment is possible after that, but applicants may have to wait for a special enrollment period. As of 2015, to enroll, one must go online, head to the local Social Security office, or call 1-800-772-1213.