When the Medicare health insurance program launched in 1966, benefits kicked in at the then-common retirement age of 65.

Today, working past age 65 is more common. But if you expect to continue to work at age 65 and beyond, you run the risk of making a potentially crippling financial mistake with your health insurance coverage.

The two major pieces of Medicare are Part A for hospital costs, and Part B for just about everything else, other than prescription drugs (that falls under Part D). If you've worked for at least 10 years, you can sign up for Part A at 65 without owing any premium. There's no reason not to sign up for Part A at 65.

Part B is trickier because most everyone pays a monthly premium for Part B coverage. In 2021, the minimum monthly Part B premium is $148.50. That's per person. There is no household coverage under Medicare. While lower-income enrollees may qualify for premium relief, individuals with income of at least $88,000 and spouses filing a joint tax return with income of at least $176,000 will pay higher premiums.

It's totally logical if you're still working at 65 and have workplace insurance to presume you don't yet need to sign up for Part B and take on the premium costs.

That can be a costly mistake.

The insurance company that runs your company's health insurance plan may deny you coverage for all the care that falls under the umbrella of Part B of Medicare. On top of that, you will also run into a penalty for your Part B premium that will stick with you for the rest of your life.

What matters at 65 is how many employees are at your business.

  • You work for a firm with at least 20 employees: Good news, assuming your employer offers coverage at age 65, you can stay on the company plan and it will pay your covered expenses.
  • You work for a firm with fewer than 20 employees: You need to sign up for Medicare Part B right when you become eligible around your 65th birthday.

For small employers, health insurance is not the "primary" payer once someone turns 65. If you are still working at 65, your workplace health insurance is within its rights to assume that Medicare will be the primary payer for your care. And your insurer doesn't really care if you didn't sign up for Medicare. You may be denied all coverage, or the plan may agree to pay only those expenses that wouldn't have otherwise been covered by Medicare Part B.

Fried writes for Rate.com.