In most situations, to qualify for Medicare's full range of benefits, you need to be 65 and have earned the required "quarters" of coverage. This generally means you are eligible if you or your spouse (or, in some cases, your former spouse) worked — and paid Medicare and Social Security payroll taxes — for at least 10 years.
However, if you’re disabled or meet other requirements, you may also qualify — provided you are a U.S. citizen or permanent resident. (For instance, you might be able to buy into the Medicare program.)
If you are already receiving Social Security benefits, an Initial Enrollment Questionnaire (IEQ) should come in the mail to you about three months before you turn 65. (If you don't receive the mailing, contact Social Security.)
If you aren't already receiving Social Security benefits and you want to enroll in Medicare, you need to apply to the program. (Note: Medicare does not reach out and contact you when you're turning 65.) You can check your eligibility and apply for coverage through Social Security online, or by calling your local Social Security office or the Social Security toll-free number at 800-772-1213.
For more information about eligibility, check out the Medicare eligibility articles featured in the archives of AARP Bulletin's Ask Ms. Medicare.