A recent look from Fidelity Investments revealed that last year's jumps in the Dow Jones Industrials (DJINDICES:^DJI) and the 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.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.