+1 on what Owlie said. The Snell Memorial Foundation, a disinterested nonprofit that tests and certifies helmets, recommends a five year replacement interval (if a helmet never gets dropped or crashed before that). Doctors' organizations, also disinterested as far as selling helmets, have adopted that recommendation.

My inclination is also that three years is a little much ... but on the other hand, it's usually the highly perforated helmets that carry the three year recommendation. It certainly makes sense that a helmet that has a greater amount of surface area exposed to acids, oils, ozone and UV, and less structural foam that isn't exposed to all that degradation, would have a correspondingly shorter lifespan.

Bottom line, whether a helmet lasts three years or five, brain injuries are forever. Mostly I replace mine after three and a half or four years. Even the most expensive helmets are cheap insurance in comparison, though I'm fine with helmets on the middle to low end of the price range.