In most cases, if you don't sign up for Part B when you're first eligible, you'll have to pay a late enrollment penalty. You'll have to pay this penalty for as long as you have Part B. Your monthly premium for Part B may go up 10% for each full 12-month period that you could have had Part B, but didn't sign up for it. Also, you may have to wait until the General Enrollment Period (from January 1 to March 31) to enroll in Part B. Coverage will start July 1 of that year.
Usually, you don't pay a late enrollment penalty if you meet certain conditions that allow you to sign up for Part B during a Special Enrollment Period.
If you have limited income and resources, your state may help you pay for Part A, and/or Part B. You may also qualify for Extra Help to pay for your Medicare prescription drug coverage.
Example
Your Initial Enrollment Period ended December 2016. You waited to sign up for Part B until March 2019 during the General Enrollment Period. Your coverage starts July 1, 2019. Your Part B premium penalty is 20%, and you’ll have to pay this penalty for as long as you have Part B. (Even though you weren't covered a total of 27 months, this included only 2 full 12-month periods.)