You have the right to choose not to sign up for Part B during your seven-month initial enrollment period (IEP) at age 65. But as a retiree whose health insurance comes from a former employer instead of a current one, you should be aware of the consequences if you delay Part B enrollment beyond 65 and then decide sometime in the future that you want it:
Even with good retiree health benefits when they retire, people do sometimes find that they eventually need Medicare benefits. Retiree benefits are rarely set in stone; the employer is always free to change or terminate them. The premiums might become too expensive to afford on a fixed income. And if the retiree dies, the retiree health benefits may not be continued for the surviving spouse.
Note that even if you do work beyond 65 but rely for health care on retiree benefits instead of insurance provided by a current employer (your own or your spouse’s), you will still face the same consequences if and when you come to sign up for Part B. You will not be entitled to a special enrollment period or avoid late penalties.