Helmet makers and independent testing facilities suggest replacement at those times. Some could say maybe it is a tinge of marketing but it is a safety device designed to protect the most important thing your head which contains the brain. Parts of the helmet can degrade from sweat, sun damage and heat. Also if your helmet has been in a crash or has been dropped it is time to replace. A helmet is a one time use item once it has that impact it has done its job and is time to be gotten rid of.