When you're used to the government taking money from you your whole working life, getting a monthly Social Security check feels like a sweet reward, regardless of how much it is. But those who start claiming Social Security as soon as they're eligible just because they can could be making a costly mistake.

It's true that the government sets the formula for how your Social Security benefit is calculated, but you influence how much it actually is by when you decide to begin benefits. Starting too early based on your life expectancy could cost you tens of thousands of dollars, as I explain below.

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How the age you start Social Security affects your benefits

The Social Security benefit formula looks at your average monthly income over your 35 highest-earning years, with adjustments for inflation. This is known as your average indexed monthly earnings (AIME), and it's used to calculate your benefit at your full retirement age (FRA), which is 66 if you were born between 1943 and 1954. For those born after that, it increases by two months every year until it reaches 67 for adults born in 1960 or later. 

You must wait to claim benefits until this age if you want the full amount you're entitled to based on your work record. You can start benefits as early as 62, but then you'll only get 70% of your scheduled benefit per check if your FRA is 67 or 75% if your FRA is 66. You can also delay benefits past your FRA and your checks will increase every month until they reach the maximum benefit at 70. This is 124% of your scheduled benefit per check if your FRA is 67 or 132% if your FRA is 66. 

Let's look at an example to see what kind of a difference your starting age can make. The average Social Security benefit for retired workers as of January 2020 is $1,505.50 per month. If you have an FRA of 67 and you live until 90, that adds up to a lifetime benefit of $415,518, plus cost-of-living adjustments (COLAs). If you chose to start at 62 instead, you'd only get $1,054 per check, but you'd claim benefits for five more years. Still, you'd only end up with a lifetime benefit of $354,144, plus COLAs. That's a difference of $61,374. 

If you delayed benefits until 70, you'd only get benefits for 20 years, but you'd get $1,867 per month. That adds up to a lifetime benefit of $448,080, plus COLAs. This is $32,562 more than what you'd get if you'd started at your FRA, and $93,936 more than you'd get if you started benefits at 62.

Obviously, this is just one example. The difference in lifetime benefits will vary depending on your FRA, the size of your checks, and how long you live. But this example illustrates how much you can cost yourself if you start claiming benefits early just because you can.

How to decide when to start Social Security benefits

If you're angling for the largest lifetime benefit, you're usually better off delaying benefits as long as possible unless you have a terminal illness. In that case, claim benefits as early as possible so you get what you can before you die. But if you think you'll live into your late 80s or beyond, delaying benefits will most likely result in larger checks.

You can estimate your lifetime benefit using the same steps that I did above. First, figure out your estimated Social Security benefit by creating a my Social Security account. It should list your estimated benefit at 62, your FRA, and 70. Multiply each of these estimated benefits by 12 to get your annual benefits. Then, multiply these amounts by the number of years you expect to receive benefits based on your estimated life expectancy. This should tell you roughly how much you'll get over your lifetime. Of course, you have more choices than these three ages for starting your benefits. You can start them at any time between 62 and 70, but the later you begin, the more you'll get per check. 

Some people have to start claiming Social Security early because it's the only way they can afford to make ends meet. This isn't ideal, but it's something you'll have to weigh as well when deciding when to start benefits. Starting early might be the smart play in this scenario because if you don't, you might end up taking on costly debt to cover your living expenses that could negate some or all of the additional Social Security benefits you would've gotten by waiting. 

Play around with a few different scenarios to see which starting age best fits in with your retirement plan and offers you the most money overall. Review this again before you claim benefits to make sure it's still the right decision.