Plan to replace your helmet if it has been involved in an accident. Some helmet manufacturers will inspect and, when possible, repair a damaged helmet. If your helmet has been dropped and you think it might be damaged, you may want to take advantage of this service.
Most helmet manufacturers recommend that, under normal use, you should replace your helmet every two to four years. If you notice any signs of damage before then, replace it sooner. As mentioned above, fiberglass helmets may crack or break if dropped. And, if you see any light brown spots near the rivets of a polycarbonate helmet, it is probably damaged and should be replaced.
Why replace a helmet every few years if it doesn't appear damaged? Its protective qualities may deteriorate over time. The interior padding compresses, offering less protection. The chin strap may fray or loosen at its attaching points. And the shell may be chipped or banged. Probably the best reason, however, is the constant improvements in design and protective qualities of helmets. Chances are the helmet you buy in a couple of years will be better than the one you have now - even if the cost is about the same.
Since 1974, all helmet manufacturers must stamp the month and date of production on the helmet's chin strap. If you can't remember when you bought your present helmet, just check the strap. If there's no date at all, you definitely should replace your helmet now.