Many seniors routinely rely on Social Security to cover their bills in retirement. But that's a big mistake.

Social Security will only replace about 40% of the average worker's pre-retirement income. And most retirees need roughly twice that much income to maintain the lifestyles to which they've become accustomed.

Compounding the issue is the potential for Social Security cuts. Once the program's trust funds run out of money, Social Security may have to slash benefits to address a revenue shortfall.

A somewhat older person with a serious expression is examining a document.

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And the timing of those potential benefit cuts isn't so far off. The program's trustees recently estimated that its trust funds could be depleted in roughly a decade, putting seniors in a tighter financial spot sooner than many may be anticipating.

It's for these and other reasons that saving independently for retirement is so important. But in a recent survey by First National Bank of Omaha, only 17% of Americans are prioritizing retirement savings. And that means a lot of people risk falling short financially during their senior years.

Why time is of the essence

A good 65% of Americans worry they won't manage to retire by age 65, which isn't full retirement age for Social Security, but it is when Medicare eligibility kicks in. In spite of that, most of the people who responded to the survey above aren't making saving for retirement a top priority this year.

But there's a danger to putting off retirement savings: You lose out on time to benefit from investment gains. When savers sock funds away in an IRA or 401(k) plan, that money can, and should, be invested for added growth. And the longer your investment window, the more wealth you stand to accumulate.

As an example, imagine you're able to part with $400 a month for retirement savings purposes. Save and invest that sum over a 40-year window, and you'll be looking at a nest egg worth over $1.2 million if your IRA or 401(k) delivers an average annual 8% return, which is a bit below the stock market's average. (Of course, that return assumes you'll invest your savings heavily in stocks, which is a smart thing to do when retirement is many years away.)

But watch what happens when you whittle that window down to 30 years: Your ending balance shrinks to $544,000. And with a 20-year investment window, it drops all the way down to $220,000.

That's why prioritizing retirement savings is essential, and the sooner you do so, the better off you'll be. You may need to shift some expenses around to make consistent IRA or 401(k) contributions possible. And to be clear, right now, carving out funds for a retirement account is far from easy, especially with living costs being up across the board as a result of rampant inflation.

But if you wait too long to build yourself a nest egg, you'll risk a scenario where you don't have enough retirement income to pay your bills, especially if Social Security benefit cuts happen to a more extreme degree than anticipated.