You don't have to sign up for Medicare at 65. However, it might be a wise idea to at least get Part A -- since it's likely free. The Medicare tax through payroll taxes helps pay for that Medicare program.
Part A doesn’t usually have a premium. So, even if you have another health plan that you like, you can still sign up for Part A, which can help with hospitalization costs.
Medicare and other insurers have something called coordination of benefits (COB). COB figures out which plan pays first for coverage if you have more than one plan.
For instance, if your employer has more than 20 employees, the COB will consider Medicare coverage a secondary payer. If your company has fewer employees, your Medicare coverage is primary and the employer is secondary.
The primary insurer pays initially and then the secondary payer covers its portion up to 100% of the total cost.
Having more than one type of health insurance is called “dual insurance.” Having dual insurance can help reduce out-of-pocket costs but also means you may have to pay two premiums. Plus, you could face two separate deductibles.
You want to run all of those numbers to see whether keeping other coverage makes sense for you when you become eligible for Medicare.