Warren Buffett wrote:
I believe that over any extended period of time this category of investing will prove to be the runaway winner among the three we’ve examined. More important, it will be by far the safest.
In that sentence, "this category of investing" refers to stocks, and the other two are cash and gold. I agree with this conclusion except to point out that it does depend on how you define "any extended period of time." If you're pessimistic you can cherry-pick a 10-year period to demonstrate that a portfolio built around index funds grew less than the interest on a CD of the same value. If you're optimistic you can find plenty of 10-year periods that show just the opposite.
For someone in their 20s it's pretty much a no-brainer, as I've stated above: invest in equity. But as you get older the abstractness of your future scenario gradually gets replaced by the reality of the actual scenario you are living.
There's a guy on another financial forum I frequent (for Canadians) who's retired and still has most of his investments in index funds. Setting aside the fact that you're supposed to start shifting into less volatile investments as you approach retirement age, this guy is hurting because has seen very little growth in his portfolio over the past decade. The scenario hasn't played out as expected for him. He's been holding onto his index funds too long in the hopes that the market will pick up again and they will regain some of the value they've lost, but no dice so far, and his retirement plans have gone out the window.
Over the past five years, the three main index funds he's invested in have returned +0.73%, -1.44%, and -7.99%. If he was in his 20s this wouldn't matter at all, but he's in his 60s and it matters a lot.
So the lesson here is that volatility doesn't matter when you're young, but as you get closer to needing the money it matters more and more. And as you get older, the actual scenario that plays out in the market becomes more and more important than the general rule that stocks are the best performers in the long term. So you can "set it and forget it" in your 20s and 30s, but once you hit your mid 40s you should start paying closer attention. You can have great growth for 20 years, but if it's erased by one bad decade during your 50s, that won't leave you in a good position in your 60s.