Social Security serves as a lifeline for millions of seniors who need help paying the bills. There's just one problem: The program alone cannot sustain retirees. Rather, those looking to live a reasonably comfortable lifestyle in retirement must take steps to save on their own during their working years, or risk falling short on their bills once they're older.

It's therefore concerning to learn that 21% of working adults currently have no savings to show for, according to new data from Northwestern Mutual. And if they don't start taking steps to build their nest eggs, they're going to run into trouble once their careers wrap up and their paychecks disappear.

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Retirement: It's more expensive than you'd think

Many workers assume that once they leave their jobs, their living costs will magically shrink. The reality, however, is that a large number of common expenses stay the same or even increase in retirement. Take groceries, for example. Just because you aren't working doesn't mean you don't need to eat, so there's no reason to think it'll cost less money to feed yourself when you're older.

Now let's talk about healthcare, because that's an expense that typically goes up in retirement. Not only is Medicare not totally free for seniors, but there are a number of services it doesn't cover at all, like dental, hearing, and vision care. Throw in copayments, deductibles, and the like, and the average 65-year-old male today is looking at spending $189,687 on healthcare over the course of retirement, not including long-term care. For women, this figure climbs to $214,565. Ouch.

Another expense that's likely to climb in retirement is leisure spending, since retirees have more free time on their hands than workers. And while entertainment certainly isn't a necessity the way healthcare is, it's fair to assume that you'll spend a bundle keeping yourself occupied.

All told, the average retiree needs about 80% of his or her former income to pay the bills, and Social Security will only provide about half that amount. Therefore, if you're not saving on your own, you're setting yourself up for a perpetually cash-strapped existence. And you deserve better.

Saving money slowly but surely

Saving for retirement doesn't need to be the sacrifice many workers make it out to be. While it's true that 401(k)s today max out at pretty high levels ($18,500 for workers under 50 and $24,500 for those 50 and over), you can get away with saving a smaller amount each month if you give yourself a decent savings window. (Besides, not everyone has access to a 401(k), and if you're saving in an IRA, you're limited to $5,500 a year if you're under 50, and $6,500 a year if you're 50 or older). However, the longer you wait to start saving, the more you'll have to contribute each month to make up for lost time.

Check out the following table, which shows what a monthly contribution of $400 might do for your nest egg over time:

If You Start Saving $400 a Month at Age:

Here's What You'll Have by Age 65 (Assumes a 7% Average Annual Return):

25

$958,000

30

$663,000

35

$453,000

40

$303,000

45

$197,000

50

$120,000

55

$66,000

TABLE AND CALCULATIONS BY AUTHOR.

As you can see, if you start saving when you're young, a steady stream of $400 monthly contributions will leave you with close to $1 million by the time you retire. Keep waiting, however, and that ending balance gets lower and lower. Therefore, if you're in your 50s and don't have any money saved to date, you'll need to do better than $400 a month. But if you do manage to max out a 401(k) for a 10-year period from, say, age 55 until 65, you'll end up with $338,000, which will certainly help cover the bills when you're older.

And in case you're wondering about that 7% return, it's more than doable with a stock-heavy portfolio. And if you have 10 years or more between when you start saving and when you're set to retire, that's ample time to ride out the market's ups and downs.

One of the biggest mistakes you can make during your working years is neglecting your future -- so don't. Start putting money aside for your nest egg immediately, even if it's a tiny amount, and work your way up over time. Otherwise, you're likely to struggle financially when you're older, and be all the more miserable for it.

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