When to Enroll

Your age-based first window lasts seven months: the three months before your 65th birthday month, that month, and the three months after. A birthday on the first shifts the window one month earlier. Enrolling before that effective birthday month starts Part B and premium Part A on its first day; signing up during or after it starts them the next month. Premium-free Part A follows different start-date rules.

If you (or your spouse) are still working and covered by a qualifying employer plan, you can usually delay Part B without penalty — and you get an eight-month Special Enrollment Period after employment or employer coverage ends, whichever happens first. The rules differ for small employers, COBRA does not count as employer coverage, and contributing to an HSA while enrolled in any part of Medicare is not allowed — three details that catch more people than any others.

Every fall, the Annual Enrollment Period (October 15 – December 7) lets eligible beneficiaries join, switch, or drop Advantage or drug plans for January 1. Advantage members get one more adjustment window early in the year. Plans change their costs and formularies every single year — which is why a plan that fit last year deserves ten minutes of review this year.

Part B generally adds 10% for each full 12-month period you could have enrolled but did not, usually for as long as you have Part B. A qualifying Special Enrollment Period or Medicare Savings Program can prevent this penalty. Part D can add 1% of the national base beneficiary premium per full uncovered month after a gap of 63 consecutive days without creditable drug coverage; people with Extra Help do not pay it.