If you continue to work full or part time after turning 65, and will be covered by a qualified health plan through your employer, you are generally not required to enroll in Medicare Part B. Check with your employer. If you waive enrollment in Medicare Part B because you are covered by your employer’s insurance you are not subject to a Part B late enrollment penalty.
There are separate rules regarding avoiding a Medicare Part D late enrollment penalty if you continue to be covered by your employer’s insurance. In this event you must have prescription coverage as good as or better than what you could get under Medicare Part D. Your employer is required to give you proof of this each year.
It is important to note that coverage under COBRA from a prior employer will not prevent a Medicare Part B late enrollment penalty.