According to medicare.gov If you’re retired and have Medicare and group health plan (retiree) coverage from a former employer, generally Medicare pays first for your health care bills, and your group health plan coverage pays second.
There are certain times of the year when you can sign up or change how you get your coverage. Every year you can check and make a change to your Medicare plan from October 15 to December 7 called the “Annual Election Period” annual enrollment period.
If you’re still working at 65, and you have coverage under a group health plan through an employer, then you don’t have to enroll in Medicare right now. But if your employer has less than 20 employees, you need to take Medicare Parts A and B, because that will be your primary insurance.