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Most Americans have heard of Medicare and have come to associate it with senior healthcare. But how much do you know about this key program? As of April 2017, there were nearly 58 million people enrolled in Medicare, which is close to 20% of the total U.S. population. Even if you're not signing up for Medicare anytime soon, there are certain facts about the program you should be aware of. Here are just a few to commit to memory.

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1. It has several distinct parts

People tend to think of Medicare as a single program, but in reality, it has several different parts that work independently of one another. Here's a basic rundown:

  • Medicare Part A covers hospitals, nursing facilities, and home health services.
  • Medicare Part B covers preventive services such as doctor visits, diagnostic tests, and medical equipment.
  • Medicare Part C is its own separate health plan, offering additional benefits not covered by traditional Medicare. It's also known as Medicare Advantage.
  • Medicare Part D covers prescription drugs.

As an enrollee, you'll get the option to choose your Part C or Part D plan based on your specific needs, keeping in mind that different plans offer varying levels of coverage.

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2. Eligibility begins at 65 -- but you can sign up before then

Though Medicare coverage doesn't actually kick in until age 65, your initial enrollment period begins three months before the month of your 65th birthday. If you're already retired, or planning to retire by that point, it pays to sign up early.

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3. It's not free

Though most people don't pay a premium for Medicare Part A, the remaining parts come at a cost. Currently, the standard premium for Part B is $134 per month, but that figure is income-dependent, so if you're a higher earner enrolled in Medicare, you could see that number rise.

Part D, meanwhile, is a bit more complicated. Though the average monthly premium in the country this year is $34 a month, your total Part D costs will depend on factors such as:

  • The plan you use
  • The prescription drugs you take
  • The extent to which your medications are purchased or offered in-network
  • Your income (keeping in mind that higher earners get surcharges tacked onto their standard premiums)

Then there's Medicare Advantage, whose price tag will depend on the specifics of your plan. While some Advantage plans offer low premiums, the flip side is that you'll pay more in copays and deductibles. That's why you'll need to research your costs thoroughly before making a decision.

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4. Enrolling late will cost you

Your initial Medicare enrollment window is seven months long, beginning three months before the month you turn 65 and ending three months after the month you turn 65. That's a pretty decent chunk of time to sign up for coverage, so be sure to take advantage. If you don't, you could end up paying more for Medicare -- for life. For every 12-month period you neglect to sign up for Part B, for example, you'll face a 10% increase in your monthly premium costs. And if you go 63 days or more without a Part D plan, you'll face late-enrollment penalties as well.

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5. It won't cover everything

Because so many seniors rely on Medicare, it might seem like the program truly covers all of their healthcare needs. But in reality, there are a host of common services that Medicare won't pay for. These include:

  • Dental care, including dentures
  • Vision care
  • Hearing aids
  • Routine foot care
  • Cosmetic procedures

That's why it pays to consider a Medicare Advantage plan. Not only does Medicare Advantage cover dental and vision services, but it also typically includes a drug plan, which means you won't have to worry about separate coverage for your prescriptions. Read up on the benefits of Medicare Advantage, because a Part C plan could end up being the best choice for you.

Medicare is a valuable but complex program that can be tricky to navigate. The more you learn about Medicare and how it works, the better positioned you'll be to secure the right coverage.

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