A recent look from Fidelity Investments revealed that last year's jumps in the Dow Jones IndustrialsĀ (DJINDICES:^DJI) and theĀ S&P 500 (SNPINDEX:^GSPC) have helped retirement investors enjoy big gains in their 401(k) accounts. But it also showed that young investors are making a huge mistake that could cost them in the long run.

In the following video, Dan Caplinger, The Motley Fool's director of investment planning, looks at the survey and its findings that 41% of those ages 20 to 39 who left their jobs in 2013 cashed out of their 401(k) plans, with an average balance of $16,000. Dan notes that it's tempting to take that money and use it for immediate needs, but the negative impact on your finances can be huge. Not only could you pay taxes and penalties of around $5,000 on a withdrawal of that size, but Fidelity also found that you'll miss out on an estimated $471 in monthly retirement income. Dan concludes that young investors have to have the discipline to roll over old 401(k) balances to new retirement accounts or to leave them alone to avoid this huge mistake.

Think young
It's no secret that young investors don't always make the right decisions with their money, but your best investment strategy is to buy shares in solid businesses and keep them for the long term. In the special free report "3 Stocks That Will Help You Retire Rich," The Motley Fool shares investment ideas and strategies that could help you build wealth for years to come. Click here to grab your free copy today.

Dan Caplinger and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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