More than 335 million people live in the U.S. Around 65 million of them receive health insurance coverage from Medicare. Of those, roughly 57 million are Americans ages 65 and older.

There's arguably no more important time of year for these 57 million individuals with respect to their Medicare coverage than right now. The Medicare enrollment clock is ticking. Here are three important things every senior needs to know.

An alarm clock on the stand next to a sleeping person.

Image source: Getty Images.

1. When open enrollment periods end

First, it's critical to understand that there are several parts to Medicare:

  • Part A covers inpatient hospital stays, hospice care, and certain short-term skilled nursing care.
  • Part B covers services including doctor visits, ambulance services, lab tests, and medical equipment. Medicare beneficiaries can also purchase Medicare Supplement (also known as Med Supp or Medigap) plans from private insurers that offer additional benefits.
  • Part C includes Medicare Advantage plans offered by private insurers that cover everything covered by Parts A and B (together referred to as Original Medicare) and can feature other benefits such as dental, prescription drug, and vision care as well.
  • Part D covers prescription drugs, with plans available from private insurers.

Seniors can sign up for Part A and Part B as soon as three months before turning 65 through three months after turning 65. However, Medicare also offers an open enrollment period each year between Oct. 15 and Dec. 7. Anyone who missed enrolling during their initial eligibility window or who wants to make changes to their Medicare Supplement or Medicare Part D plans can do so during this period. For those already enrolled in a Medicare Advantage plan, there's a different open enrollment period for making plan changes or switching to Original Medicare that extends from Jan. 1 through March 31 each year.

2. What happens if you do nothing

In most cases, doing nothing during Medicare open enrollment periods isn't a problem. Your coverage, whether it's under Original Medicare, Med Supp, Medicare Advantage, or Medicare Part D, will typically continue uninterrupted.

However, there's a good chance that the premiums could change. It's also possible that other aspects of your benefits, such as out-of-pocket maximums, could be different. Because of these factors, reading through any updates to your current Medicare policies is a smart idea.

From time to time, private insurers who offer Med Supp, Medicare Advantage, and/or Medicare Part D prescription drug plans could withdraw from certain markets. You should receive an official notification from your insurer if such a change affects you. In this event, you'll want to take advantage of the open enrollment period to enroll with another plan.

3. How to compare your Medicare options

Selecting the best Medicare options to meet your healthcare needs can be complicated. Fortunately, there are several websites that help compare plans, including the Medicare Plan Finder tool on Medicare.gov.

Costs will, of course, be a key consideration in choosing a Medicare plan. Keep in mind, though, that premiums reflect only part of the total costs of coverage. Out-of-pocket deductibles and maximums are also important to check out.

Med Supp, Medicare Advantage, and Medicare Part D prescription drug plans offered by private insurers could have limited provider networks. Look to see if the healthcare providers you prefer are in those networks. Some plans could allow you to go out of network, but it's likely that you'll have to pay more to do so.

Finally, the Centers for Medicare and Medicaid Services assigns ratings to all Medicare Advantage and Part D plans. These ratings range from one star at the lowest to five stars at the highest and are based on a wide range of quality and performance metrics. Using these star ratings can help you in selecting a plan.