This is a seven-month period that starts three months before your 65th birthday month and lets you enroll in Medicare for the first time. If you are under 65 and you’re receiving Social Security disability benefits, you’ll become eligible for Medicare starting on the 25th month of you receiving your Social Security benefits. If you become eligible for Medicare in this way, you will be able to enroll in a Medicare Advantage plan three months before your month of eligibility.