If you want to be truly wealthy when you retire, there's a simple thing you have to do. Sadly, though, many people have missed their best chance to become a millionaire -- and a new generation of investors are on a path to join them.

The most valuable asset any investor can count on is time. You may dream of finding 100-bagger stocks that will take your modest investment and make you rich without any effort. But unless you're destined to become the next Warren Buffett, you can't rely on that fantasy to make you rich. However, if you have time on your side, you can endure bear markets like 2008's financial crisis, weathering the storm and making it through to the recoveries that inevitably follow. And over the long haul, you'll be able to follow the general upward path that stocks have given investors for decades, and even take advantage of temporary downturns to load up with stocks on the cheap.

Let time work for you
As much as the market meltdown two years ago hurt investors with sizable portfolios, the damage it did to the mindset of young investors may have an even greater negative impact. According to a survey last year from the Investment Company Institute, just 22% of investors under age 35 want to take substantial risks in the financial markets. A separate survey showed that more than half of young investors are more conservative now than they were in 2009, and those young investors have made the biggest shift toward less aggressive investing strategies as a result of the financial crisis.

That may be the latest reason why young investors will be slow to start investing toward their long-term goals, but it's definitely not the only one. Consider some of the excuses that people have traditionally used to put off investing:

1. I just started my career; why think about retirement?
When you first join the workforce, money is almost always tight. With competing demands ranging from student loans and other debt to buying a first home or starting a family, retirement is easy to put on the back burner.

But starting early gives you an extra 10 to 20 years for your money to grow. Even if you can't afford to save much, by the time you retire, small amounts will have grown to large amounts. And more importantly, starting the savings habit early makes it easier to stick to your financial plan later in life.

2. The market's too risky.
There are plenty of reasons why now might seem like a terrible time to invest. The market is up more than 90% from its lows less than two years ago, and many stocks have seen even bigger gains. Stocks that came back from the brink of ruin, including Wynn Resorts (Nasdaq: WYNN) and Sirius XM Radio (Nasdaq: SIRI), have seen stock prices multiply as much as 30-fold from those dark days in early 2009. Now, they're not just surviving; they're thriving, with the casino industry strengthening and Sirius turning profitable. A lot of easy money has already been made, and now many stocks seem expensive.

Moreover, some hot markets are starting to turn over. Emerging-market stocks are seeing pressure for the first time in a while -- with the iShares FTSE China 25 ETF (NYSE: FXI) down more than 10% from its November levels and India Fund (NYSE: IFN) off almost 12% since the end of 2010.

The superhot precious metals sector is also seeing a reversal. SPDR Gold Trust (NYSE: GLD) saw the biggest one-day outflow in its history yesterday. Mining stocks have plummeted in the wake of the recent fall in precious-metals prices, with Silver Wheaton (NYSE: SLW) having lost a quarter of its value just since the beginning of the year, and Freeport-McMoRan Copper & Gold (NYSE: FCX) off 13% in less than two weeks.

That said, there are always risks in the market. If you're always convinced that stocks are either too risky or not cheap enough, then you'll never invest. The key is to find investments that have the best long-term prospects no matter what may happen in the next few weeks or months.

3. I can't afford to invest.
Coming up with money to save is never easy. If you can only manage to come up with small amounts to invest, you might well think it's not worth it.

But over 40 years, it's not unreasonable to expect your money to double in value four or five times, meaning that $1 saved today will be worth $16 or $32 when you retire. That's not chump change; even if you can only set aside $25 or $50 a month right now, it could grow to thousands by the time you retire.

Don't wait another minute
Procrastination is always easier than taking action. But you can't afford to wait any longer. Starting to save and invest takes effort, but it's the best move you can make with your money.

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Fool contributor Dan Caplinger expects to retire a millionaire -- eventually. He doesn't own shares of the companies mentioned in this article. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Fool's disclosure policy looks forward to the food at those free investing seminars it makes fun of.