I do not think it is automatic. See this:
Medicare enrollment begins three months before your 65th birthday and continues for 7 months. If you are currently receiving Social Security benefits, you don’t need to do anything. You will be automatically enrolled in Medicare Parts A and B effective the month you turn 65. If you do not receive Social Security benefits, then you will need to sign up for Medicare by calling the Social Security Administration at 800-772-1213 or online at http://www.socialsecurity.gov/medicareonly/. It is best to do it as early as possible so your coverage begins as soon as you turn 65.
So if you started receiving SS before 65 you get automatically enrolled, if not then you have a 10 month window where you MUST enroll in Medicare. My point was, that once you turn 65 you are required to enroll in Medicare. It is mandatory.