I researched online and found just one helmet manufacturer who had a recommendation when a helmet should be replaced. On its website, Bell recommends that their helmets be replaced every three years but the website gives no reason why. Giro gives no recommendation on their website.
All the articles I read agreed that heat will deteriorate a bicycle helmet and make it brittle. Although there are some skeptics, most manufacturers and other experts state that UV (ultraviolet) light will also damage helmets. Everyone agrees that if your helmet is impacted in a crash it should be replaced.
Most, if not all, bicycle helmet manufacturers have a crash replacement program. If your helmet is damaged in an accident you may be eligible for a credit toward the purchase of a new helmet. Check online with your helmet’s manufacturer for their replacement program details. I suggest you keep the purchase receipt for your helmet since proof of purchase is generally required to participate in replace-ment programs.
Recently, I visited my 3 favorite local bicycle shops and asked them how often they suggest helmets be replaced. Two shops stated every 2 years and the third shop told me every 2 -3 years. They all mentioned the intense Arizona heat and sun as factors in helmet deterioration.
Ultimately, the decision is yours. If your helmet is impacted in a crash you definitely should replace it. Otherwise, my opinion is that replacing a helmet every 4-5 years is prudent.