It depends. If you or your spouse have a large group employer or union coverage based on active work, then you might qualify to put off taking Part B and paying its premium until you no longer have this insurance. See the previous question on employer coverage.
When your active work employer group coverage (not COBRA) ends, you will have an eight-month special enrollment period (SEP) in which you can sign up to have your Part B begin. It can start without a lapse in coverage or a penalty on your Part B premium. Again, be sure to confirm with Social Security now that you will qualify for the special enrollment period. Document the contact – keep a record of the date, time, name of the representative, and information.
If you delay Part B and do NOT qualify for a special enrollment period, you will be able to enroll only in the General Enrollment Period, which is every year from Jan. 1 to March 31. Coverage begins July 1 that year. In addition, you will be charged a 10 percent premium penalty for every year that you were eligible to have Part B but did not. That penalty will be added to your Part B premium for as long as you have Medicare.