You need to change the bicycle helmet if it is harmed in the accident or it serves you for 5 years or more. Producers says that after 5 years of using the glue and connecting systems may not protect you as good as when it is a new product. The average helmet life is 3-5 years. The cause is foamed polymers, the material which helmet is made of, is destroying under the ultraviolet rays impact. If you are riding the whole year around you need to change the helmet every 2-3 years, so the polymers are bad at the temperature change and can lose their protective feature.