Most people age 65 or older are eligible for free Medicare hospital insurance (Part A) if they have worked and paid Medicare taxes long enough. You should sign up for Medicare hospital insurance (Part A) 3 months before your 65th birthday, whether or not you want to begin receiving retirement benefits.
Thereof, is it mandatory to sign up for Medicare at age 65?
Those who receive Social Security benefits are automatically enrolled in Medicare when they turn 65. There is no way to opt out of Medicare once you are 65 if you receive Social Security. Most individuals are unwilling to forfeit their Social Security benefits, and thus accept the enrollment into Medicare.