Medicare eligibility begins at the age of 65, but your initial enrollment window spans seven months. Specifically, it begins three months before the month of your 65th birthday and ends three months after the month of your 65th birthday.

Some people, however, miss their initial enrollment window. If that happened to you last year, you have another chance to sign up for Medicare -- but you'll need to get moving.

How Medicare's general enrollment period works

If you missed your initial Medicare enrollment window, you can still sign up for coverage during the program's general enrollment period, which runs from Jan. 1 though March 31 each year. To be clear, this isn't open enrollment where you can choose to switch your Part D drug plan -- that open enrollment takes place in the fall for existing Medicare enrollees. Rather, Medicare's general enrollment window allows you to first sign up for coverage that will then take effect on July 1.

Medicare enrollment form

Image source: Getty Images.

What that means, however, is that you could still be looking at a dangerous gap in coverage. Say you missed your initial enrollment period last year but also weren't covered at the time by an employer plan. You may have decided to go without health insurance for a bit because you weren't experiencing issues and wanted to avoid the costly monthly premiums associated with Medicare Part B (Part A is generally free for enrollment).

But what if you're having health problems now? While you can sign up for Medicare, your coverage won't kick in until July, which means you may be on the hook for costly medical expenses that can't wait between now and then.

Be sure to sign up for Medicare on time

Delaying your Medicare enrollment and falling back on the general enrollment period can be costly. The reason? You'll be charged a 10% lifelong surcharge on your Part B premiums for each yearlong period you were eligible for coverage but didn't get it.

That's why it's a good idea to sign up during that initial seven-month enrollment period unless you happen to be covered by a health insurance plan through an employer, either your own or your spouse's. In that case, you won't get penalized for signing up outside of your initial enrollment window.

Rather, you'll be given a special enrollment period lasting eight months, during which time you can sign up. That period will begin the month after you separate from your employer or you lose your employer health coverage -- whichever happens first.

Another thing you should know is that you can sign up for Medicare without Social Security. Medicare, as mentioned earlier, begins at age 65, whereas full retirement age for Social Security purposes kicks in later.

Medicare has a number of different enrollment periods, and keeping track of them can be confusing. If you missed your initial enrollment window, now's your chance to sign up for Medicare and secure coverage by July. But don't wait; the longer you go without coverage, the higher a Part B surcharge you stand to incur, and the more you'll put yourself at risk of racking up costly bills in the absence of proper insurance.