I'm in the mid-20s, so I don't touch them. Over the long haul they will be positive.
If you're talking about stock in an individual company or index and you're not looking for an immediate cash out, I wouldn't touch them. It's inevitable that we'll see a dip, we've had a strong market for a while, a correction is coming. The question isn't if, it's when. What happens after a correction though? The rebound.
If however you are looking for some day trades or short term investment, there is no magic answer. Sometimes you have to take a loss that you feel comfortable taking because it's better than losing even more. This point is different for everyone. My approach to individual stocks is simple, if I'm putting money into a stock, I'm ok losing that money. In my mind, when I put money into a stock I'm placing it out of reach so I effectively don't own that money anymore.