Signing up for Medicare is necessary for those who don't qualify automatically for it. As a rule, anyone under 65 years of age with no disabilities needs to apply for Medicare during a seven-month enrollment period that begins three months before one's 65th birthday.
Persons who are already getting retirement benefits from Social Security or the Railroad Retirement Board (RRB), who have Amyotrophic Lateral Sclerosis (ALS, or Lou Gehrig's Disease) or who live in Puerto Rico and receive benefits from Social Security or the RRB automatically get Medicare benefits. Those who have End Stage Renal Disease are also qualified, but their receiving benefits is not automatic. Most people apply for Medicare when they're about to turn 65; at that time, they need to decide between Part A only or both parts; Part B bears a premium cost, whereas Part A does not.