Social Security is or will be a vital source of retirement income for the vast majority of Americans. Indeed, it makes up 90% or more of the income for a fifth of married elderly Social Security recipients and for close to half of unmarried ones.

Thus, it's rather critical to get familiar with the program and to know enough to make smart Social Security decisions, such as when you will start collecting benefits. You can do so as early as age 62, but it's not always smart to do so -- here are three reasons why.

No. 1: You can collect more by delaying

A key reason not to start collecting at age 62 is that your checks will grow bigger the longer you put off starting, until age 70. For every year beyond your full retirement age (that's 66 or 67 for most of us) that you delay starting to collect, your benefit check will swell by about 8%. So delaying from age 67 to age 70 will inflate those checks by about 24%. Meanwhile, collecting early shrinks those checks, by as much as 30% if your full retirement age is 67 and you start collecting at 62. That's not quite as bad as it seems, though, because while the checks will be smaller, you'll collect many more of them.

Here's a quick look at how your benefits change depending on when you start collecting -- if you were born in 1960 or later and therefore have a full retirement age of 67:

Take a look at the chart here.

No. 2: You might live a long time

While it's true that starting early gets you smaller checks but many more of them, it still won't be worth starting early unless you live an average-length life or a shorter one. The system was designed to be a wash no matter when you start collecting, for those with average-length lives.

But your family might feature lots of 90-something elders, and your health might be excellent. If so, delaying as long as possible, up to age 70, can be a smart move. Your checks will be maximized and you'll collect quite a few of them due to what should be a long life.

The Social Security Administration itself has noted that, "About one out of every three 65-year-olds today will live past age 90, and about one out of seven will live past age 95."

No. 3: Your spouse might benefit if you wait

Here's another reason to not start collecting early: Your spouse might benefit if you delay. This is very much the case if your earning history is much stronger than his or hers. Why? Well, understand that when one of you dies, the survivor gets to collect either Social Security benefit -- their own or their late spouse's -- whichever is larger. If the higher-earning spouse delays collecting benefits as long as possible, ideally until age 70, then their benefits will be maximized, and whichever spouse survives will get to collect as much income as possible from the program.

It's worth learning more about how your marriage might factor into your Social Security decisions.

The more you know about Social Security, the more you can get out of it. Don't leave valuable retirement income on the table.

This article originally appeared in the Motley Fool. The Motley Fool has a disclosure policy.

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