Short answer: It depends on employer size. If your (or your spouse’s) employer has 20+ employees, that plan usually pays first and you can delay Part B without penalty. If the employer has fewer than 20 employees, Medicare typically pays first, so you generally should enroll in Part B at 65.
Whether to take Part B at 65 while still working hinges on the size of the employer providing your coverage. At an employer with 20 or more employees, the group plan is primary and Medicare is secondary, so you can usually delay Part B (avoiding its premium) and pick it up penalty-free via a Special Enrollment Period when you retire. At an employer with fewer than 20 employees, Medicare is generally primary, meaning it pays first whether or not you enroll, so failing to take Part B could leave big gaps, and you typically should enroll at 65. Always confirm with the employer’s benefits administrator how the plan coordinates with Medicare before deciding.