If you delay enrollment in Part B after you have become eligible for Medicare and are not covered by other insurance such as a group plan, you may incur a late enrollment penalty. The penalty is triggered when you enroll in Part B and is based on the amount of time that has passed since your Initial Enrollment Period expired at the end of the third month following your 65th birthday month. The penalty will be applied as long as you have Part B.
The Part B late enrollment penalty is 10% of the Part B monthly premium for each full 12-month period you were without Part B following your Initial Enrollment Period.
EXAMPLE: Mr. Farnham turned 65 in September of 2011 and is not covered by a group plan. He enrolled in Part B as of July 1, 2015. His penalty begins January 2012, three months following the month he turned 65. Three full 12-month periods elapse before he enrolls in Part B in July of 2015. Therefore, his Part B late enrollment penalty is 30%.