Back in 1935, when Social Security was first established, the average life expectancy was only about 61 years. That number has been rising ever since, to 78 currently, according to Stephen Horan, head of private wealth management for the CFA Institute and co-author of "The New Wealth Management." Moreover, once you've already made it to age 65, you're likely to live until 81 or 83 years old.

A healthy diet, exercise and a history of long-lived ancestors can boost your life expectancy still higher. So even if you think you're going to live until age 85, you need to plan for the possibility you'll make it to age 95 or 100, Horan says.

"Overwhelmingly, the biggest one is the risk of outliving your money," says Ken Fisher, chief executive officer of Fisher Investments."Most people underestimate the amount of time they're going to live, and they invest as if they're going to die in 10 years."