Didn’t invest

My dumbest investment over the past decade is not having invested at all. I just let time go by without learning or gaining anything. — Eric, online

The Fool responds: That’s definitely regrettable, but you’re not alone. A Gallup poll last year found that only about 55 percent of Americans are invested in stocks.

Each year that you put off investing will cost you: Imagine, for example, that you are 40 and hope to retire in 25 years, at age 65. If you invest, say, $6,000 annually and earn an average annual return of 8 percent, you’ll end up with about $473,726. If you start a year later, investing $6,000 less, in total, and only having your money grow for 24 years, you’ll end up with $432,636 — about $41,000 less! That money would make a big difference in retirement; along with Social Security, it might help support you for two whole years.

Fortunately, all is not lost. You can start saving and investing right away. A simple, low-fee, broad-market index fund, such as one that tracks the S&P 500, is a great way to start. Over time, increase the amounts you invest by as much as you can afford. Be sure to only invest money you won’t need for at least five to 10 years in stocks, as the stock market is volatile. An introductory book on investing can help, too — try one by John Bogle or Peter Lynch.