Turning 65 in March and buried in mail. What do I actually have to do?

Almost all of it is marketing. Here is what is actually true.

If you are still working and covered by an employer plan with 20 or more employees, you can usually delay Part B without penalty. Do not let anyone rush you out of coverage that works.

What you do need to do is confirm, in writing, from your employer's HR or benefits administrator, that your coverage is considered creditable. That single letter protects you from late-enrollment penalties later.

Your Initial Enrollment Period runs from three months before your birth month to three months after. That is a real deadline, but delaying is a legitimate choice when you have qualifying employer coverage.

The mail is not a deadline. The letter from HR is the errand.

One addition — when you do eventually enroll, that is the moment to compare Part D against your actual prescription list rather than picking the cheapest premium. The cheapest premium is frequently not the cheapest year.