You become eligible for Medicare when you turn 65, as long as you have earned 40 credits from working for about ten years. If you have not yet earned all of these credits, you still qualify, but will have to pay more for Medicare coverage. The period to enroll in Medicare, known as your open enrollment period, goes from three months before the month that you turn 65 to three months after (giving everyone a seven-month window).
Those under 65 years old are only eligible for Medicare if they are receiving Social Security Disability Insurance (SSDI). Typically, the rule is that you must have been receiving SSDI for at least two years before becoming eligible to receive Medicare benefits. This two-year waiting period begins the month that you receive your first SSDI check. Exceptions do exist for those with end-stage renal disease and/or Lou Gehrig’s Disease (ALS).