Generally, no. In most cases, if you missed your Part B enrollment window, which runs from the three months before the month of your 65th birthday through the three months after the month of your 65th birthday, you will face a late enrollment penalty once you do enroll, which will be added to your premium costs for the remainder of your enrollment. The penalty equals 10% of the standard monthly premium for each 12-month period that you delayed enrollment.
If you did not enroll for Part B during your initial enrollment period, you may qualify for a Special Enrollment Period (SEP) to sign up for Part B (and/or Part A) anytime as long as you or a spouse is working and you’re covered by a group health plan through that employment. For people age 65 or over who have coverage through a group health plan, there is also an 8-month SEP which starts the month after the employment ends or the group health plan coverage ends. If you sign up during an SEP, the late enrollment penalty will not apply.
If you missed your initial enrollment period when you first became eligible for Medicare, you can only sign up for Part A and/or Part B during the General Enrollment Period, which runs from January 1 to March 31 each year. Your coverage will begin the first of the month following the month you enrolled during the General Enrollment Period. You cannot use the annual open enrollment period that runs from October 15 to December 7 to enroll in Medicare Part A or Part B. That enrollment period can only be used by people with Medicare to sign up for drug coverage or Medicare Advantage coverage, or switch coverage that you already have.