Millions of seniors rely on Medicare for health coverage in retirement, so it’s important to understand when to sign up. Medicare eligibility kicks in at age 65, but you don’t need to wait until your 65 birthday to enroll. Rather, your initial enrollment period begins three months before the month you turn 65 and it ends three months after the month you turn 65. All told, you have a seven-month window to sign up for Medicare on time and avoid late enrollment penalties.