This article was updated on October 24, 2017, and was originally published on April 12, 2015.

Tens of millions of Americans rely on Medicare to help cover the majority of their healthcare costs. Yet whether you're an older American thinking about retirement, or one of the many people who are eligible for Medicare for various medical reasons, it's important to know when Medicare starts so that you don't miss out on valuable benefits. Below, we'll take a look at the rules that talk about specific Medicare age eligibility, as well as the exception to those age-based Medicare guidelines.

The magic Medicare age: 65

The most common way that participants get Medicare coverage is when they turn 65. The initial enrollment period for Medicare begins three months before the month that contains your 65th birthday, and it continues for three months after that birthday. As a result, you have an initial seven-month period during which you can sign up for Medicare.

Clipboard with Medicare paper and stethoscope.

Image source: Getty Images.

Medicare coverage for those turning 65 isn't automatic, so you will have to take action to sign up. If you sign up during the months before your 65th birthday, then coverage for most people kicks in on the first day of your birthday month. If your birthday is on the first day of the month, however, then coverage begins on the first day of the previous month.

If you wait until the month you turn 65 or later, there'll typically be a lag before your Medicare benefits start. There's a one-month delay if you sign up during your birthday month, a two-month delay if you sign up the month after your birthday, and a three-month delay for those signing up later in the initial enrollment period. Those who sign up during the general enrollment period that runs from January 1 to March 31 have their coverage begin on July 1.

Special conditions for Medicare eligibility

Many people don't realize that, in some cases, you don't have to be 65 to start receiving benefits from Medicare. In some cases, those who qualify will get Medicare automatically, while others still have to apply.

For those under 65 who have a disability, Medicare enrollment automatically happens once you've received Social Security or Railroad Retirement disability payments for 24 months. If you don't want those benefits -- often because you're covered under a health plan that your spouse receives -- then you can take steps to get Part A hospital coverage only, choosing not to get Part B medical coverage for which you'd typically have to pay a monthly premium.

Certain disabilities get slightly different treatment. Those who have ALS, also known as Lou Gehrig's disease, can get Medicare benefits the first month that they get disability from Social Security or Railroad Retirement.

In addition, those who have end-stage renal disease are eligible for Medicare no matter how old they are. Coverage generally starts on the first day of the fourth month of dialysis treatments; but for those who take part in a home dialysis training program, coverage can start as early as the first month of dialysis. You aren't required to sign up for Medicare, though, leaving options like group coverage or marketplace health-insurance exchange policies open as alternatives.

Be smart about when to start Medicare

The rules governing when Medicare starts can seem complicated; but again, the vast majority of people can focus on getting their benefits when they reach the Medicare age of 65. By being aware of the exceptions, though, you can coordinate your Medicare benefits with other healthcare options, to make sure that you get as much financial support as you can to help you cover the costs of staying healthy.