If you are eligible for Medicare, your initial enrollment period for Part A and Part B begins three months before the month of your 65th birthday and ends three months after it. For example, if your 65th birthday is in June, your enrollment period will extend from March 1 through September 30. If you join during one of the 3 months before you turn 65, coverage will begin the first day of the month you turn 65. If you join during the month you turn 65 or during one of the 3 months after your turn 65, coverage will begin the first day of the month following the month you enroll.
Once you have Part A and Part B, you are then also eligible to enroll in a Medicare Advantage (Part C) plan and/or a Part D (prescription drug) plan. If you are already receiving Social Security benefits when you turn 65, you will automatically be enrolled in Part A and Part B. If you are not already receiving Social Security benefits and you want to enroll in Medicare, you should contact Social Security.
Part B is voluntary, but you may face a penalty for late enrollment if you do not sign up it when you turn 65, unless you or your spouse are still working and have a group health plan from the employer. Similarly, Part D is voluntary, but you may face a penalty for late enrollment if you sign up for a Part D plan after your initial enrollment period ends and you go for more than 63 days without a source of drug coverage at least as good as what Medicare Part D offers.