Couple blurbs I found:
Since helmets are made of materials which deteriorate with age, they have a limited life span and must be replaced after five years (less if the manufacturer recommends).
Snell institute recommendations are that you should replace your helmet every two years if you race, and every five years regardless. Helmets significantly more than five years old should be destroyed. The components in the helmets that protect you tend to deteriorate with time. The time starts when the helmet is made, not when the box is opened. For this reason alone, it's an excellent idea to buy your helmet mail order: the large mail order houses have very fast turnover on their stock, so you're very likely getting a very new helmet. You can check on this simply by asking if your preferred model is popular. At your local dealer, there's a good chance that the helmets have been sitting on the shelf for six to eighteen months, so a significant portion of their service life is already used up before you ever put the helmet on your head. Also, avoid buying a helmet at the closeout places for the same reason: a one or two model year old helmet has already used up a third to a half of its service life sitting around. If your helmet is older than five years, you may be fooling your wife and yourself, but you're not fooling the laws of physics. Oh, and remember, the laws of physics were discovered by physicists, but we didn't make them: they're God's laws, not ours.