The helmet should be replaced when you find one in a cooler color....especially if it's available in purple. *wink*
For some reason, the "guidelines" for helmet replacement remind me of shampoo directions...wash, rinse, repeat. The more you repeat, the more shampoo you buy....or the guideline of how many months salary should be devoted to the purchase of an engagement diamond.....where on earth did THAT come from?
ANYHOOOO, back to the original question. Like most anything else, a helmet can begin to lose effectiveness as it ages. I always followed the five year plan, though I still wear some helmets older than that.