Turning 65 is a major milestone for one big reason -- it's when you're eligible for health coverage under Medicare. But if you're still working at age 65, Medicare coverage may not be necessary.

If your job provides you with health insurance, you may still be able to sign up for Medicare and use it as secondary insurance. But that also may not be necessary. And since there's a cost to Medicare Part B, if you're still working at 65, you may decide to delay your enrollment.

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Generally, people who wait too long to sign up for Medicare risk lifelong surcharges on their Part B premiums. But there can be an exception for people who are still working at 65 and still have qualifying health coverage.

The key, however, is to make sure you're eligible for a special enrollment period to sign up for Medicare. If you aren't, a late enrollment could prove very costly.

How Medicare enrollment works

Your initial Medicare enrollment window lasts seven months, starting three months before the month of your 65th birthday and ending three months later. However, if you're still working at 65 and covered by a qualifying group health plan through an employer, you're generally able to delay enrolling in Medicare Part B without facing penalties.

Once you leave that job or lose the health coverage that came with it, you'll generally qualify for a special enrollment period that lasts eight months to sign up for Medicare. But the key to getting a special enrollment period is having qualifying group coverage during your initial enrollment window.

Qualifying group coverage usually applies to health insurance plans with 20 or more employees. However, certain types of health plans generally do not count as qualifying group coverage. These include:

• COBRA
• Retiree insurance plans
• Health Insurance Marketplace plans

If you have one of these plans and you don't want to face lifelong surcharges on your Medicare premiums, you'll need to sign up during your initial enrollment period. You generally will not qualify for a special enrollment period later.

Don't stick yourself with higher costs

If you don't sign up for Medicare on time, you could face a 10% surcharge on your Part B premiums for each 12-month period you were eligible for coverage but didn't sign up. And that surcharge isn't a one-year thing. It stays with you for life.

Medicare is expensive enough without having to pay more for Part B. So make sure you understand when you're supposed to sign up. And if you're counting on a special enrollment period to avoid surcharges, make sure your health plan actually qualifies.