The Medicare Special Enrollment Period (SEP) is for those that maintained group coverage (20+ employees) at age 65 and up and did not enroll in Part B at age 65. If you are a sole proprietor you must enroll in Medicare A and B at 65 during the Initial Enrollment Period (IEP.)
Enroll in Medicare Part A at 65 even if you are still enrolled in an employer group plan (unless you want to continue contributing to an HSA.) Once enrolled in premium-free Part A you will be assigned a Medicare number. When you retire and are ready to enroll in Medicare Part B during your (SEP) you will need your Medicare number.
The Medicare Special Enrollment Period (SEP) begins three months after the birth month at 65, which coincides with the ending of the Initial Enrollment Period (IEP.) The (SEP) will continue through active group coverage, and ends eight months after employment ends. It is highly recommended that you enroll in Medicare Part B a month before you plan to retire or the employer group coverage ends, forgetting about the eight month window. In other words, don’t put it off. Procrastination can cost you a bundle in lifetime penalties.
Under no circumstances should you elect COBRA at 65. Immediately enroll in Part A and B instead. There are potential lifetime penalties involved when electing COBRA at 65. Even if your employer subsidizes the COBRA premium to some extent I would not take the deal, instead enrolling in Medicare A and B. In the long haul you will save money and have better coverage. If you have Medicare A and B and purchase a Medicare supplement G plan you will have Cadillac coverage, the best there is. Don’t forget to buy a Part D prescription plan as well. You can shop all stand-alone Part D prescription plans in your area on the Medicare.gov website, and enroll.
You must be enrolled in Medicare Parts A and B to purchase a supplement or Advantage plan. Personally, I would never buy an Advantage plan. A true Medicare supplement policy such as the G plan will save you money over the long haul, and you will have access to many more providers.