From January 1 to March 31st each year, those who are eligible for Medicare can sign up for the first time, but you may have a late penalty if you're signing up after your initial enrollment period.
The Medicare general enrollment period is for new enrollees who do not have any type of Medicare. If you want to change parts of your existing Medicare coverage, you’ll probably need Medicare open enrollment in the fall. During general enrollment, you'll have the same options as during initial enrollment — you can sign up for Medicare Part A, B, C or D. The coverage you select will begin on July 1.
However, enrolling in Medicare after your initial enrollment period means you could pay a penalty. This isn't a one-time fee. Instead, it's an increase in your monthly costs based on how long you didn't have coverage.