This depends on your circumstances. Some folks are enrolled automatically and others have to sign up through Social Security when they become eligible.
Even if you are under the age of 65, if you qualify for Medicare due to a disability, you will be automatically enrolled. Qualifying for Medicare benefits begins after you have received Social Security Disability Insurance (SSDI) or Railroad Retirement Board disability benefits for 24 months. If you have amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig’s disease), you will automatically get Part A and Part B the month your disability benefits kick in. If you have End-Stage Renal Disease (ESRD) or kidney failure, you must enroll for yourself.
If you have been receiving retirement benefits from Social Security or the Railroad Retirement Board at least four months before you turn 65, you will be automatically enrolled and begin receiving benefits the first day of the month you turn 65.
You will need to sign up if you are turning 65 and have not already been receiving disability or retirement benefits at least four months before you turn 65.
Most Americans begin receiving Medicare when they reach the age of 65. They can first enroll during their Initial Enrollment Period, which is a seven-month window of time that begins three months before the month of your 65th birthday. During that time, you can sign up with your local Social Security office or fill out the necessary documentation through Social Security’s secure Medicare website.
When you first sign up for Medicare, you are signing up for Part A and Part B, which are referred to as Original Medicare. Many people receive Part A with no premiums, in which case it is strongly advised to sign up for Part A as soon as you are eligible. You can choose to delay enrollment in Part B with no late penalty if you or your spouse are still working at age 65 and are receiving group healthcare coverage from your employer of more than 20 people. This delay and fee waiver are thanks to a Special Enrollment Period. Once you stop employment or lose the health coverage, whichever comes first, you will have eight months to sign up for Part B.
The late penalty for Part B lasts for the lifetime of your coverage, so it is best avoided. The penalty adds up to 10 percent of the standard premium fee to your monthly premiums for every twelve months you went without signing up for Part B after you became eligible.