Social Security serves as a critical source of income for millions of retirees today. And chances are, you'll want to get the most out of the program once you're eligible to sign up for benefits. But if you end up falling victim to these three fairly common mistakes, you might kick yourself big time after the fact.

1. Not knowing what full retirement age looks like

The monthly Social Security benefit you're entitled to in retirement will hinge on how much income you earned during your 35 highest-paid years in the labor force. But that benefit won't be yours until you reach full retirement age, or FRA.

A person at a laptop raising their arms as if annoyed.

Image source: Getty Images.

FRA isn't a single age. Rather, it's dependent on the year you were born. Do yourself a favor and consult this table to see what your FRA looks like.

If Your Year of Birth Is:

Full Retirement Age Is:

1943-1954

66

1955

66 and two months

1956

66 and four months

1957

66 and six months

1958

66 and eight months

1959

66 and 10 months

1960 or later

67

Data source: Social Security Administration. Chart by author.

You're allowed to sign up for Social Security before reaching FRA -- once you turn 62, you're welcome to claim your money at any time. But for each month you file for Social Security before FRA, your monthly benefits get reduced -- for life. And that's a financial hit you may not be able to afford if you don't have a lot of savings.

2. Not realizing how much replacement income Social Security provides

Some people go into retirement feeling confident in their ability to cover their expenses on Social Security alone. But one thing to realize is that Social Security will only replace about 40% of your preretirement wages if you earn an average salary. And most seniors need more income than that to maintain a comfortable lifestyle.

Now, to be fair, you probably don't need 100% of your former paycheck once you retire. That's because you're apt to shed certain expenses, like those related to holding down a job.

But a 60% pay cut is a pretty significant one. So a happy medium is probably somewhere between 40% and 100% of what you used to earn.

Make sure your savings are substantial enough to bridge that gap. If not, you may want to consider working longer to boost your nest egg and delaying Social Security for a higher monthly benefit. For each month you delay your filing past FRA, up until age 70, your benefits get a boost.

3. Not strategizing with a spouse

It may be that you and your spouse are each eligible for Social Security based on your respective wage histories. If that's the case, you have a prime opportunity to maximize your benefits -- so don't squander it.

Rather than claim Social Security independently, sit down and devise a joint strategy. You may decide to have one of you claim Social Security at FRA so you have some income coming in while the other delays their claim for a boosted benefit for life. That's just one example. Talk through your options before jumping in.

The income you get from Social Security could really end up sustaining you to a large degree throughout retirement. So do your best not to fall victim to these giant mistakes.