When you turn 65 or otherwise become eligible for Medicare, you'll have your Initial Enrollment Period (IEP).  In most cases, your IEP begins three months before and ends three months after the month of your 65th birthday (a seven-month window).  For someone under age 65 who becomes entitled to Medicare based on disability, entitlement begins with the 25th month of disability benefit entitlement. For these individuals, the IEP begins 3 months before the 25th month of disability benefit entitlement, includes the 25th month, and ends three months after. The IEP for people with ESRD and ALS varies based on their situation.