I had also heard it was to do with every day knocks and tumbles. Even putting your helmet down on the same table after each ride will add a small amount of damage that will add up over the course of 5 years.. *shrugs* Make of that what you will but I tend to think that several years of the lid maybe rolling on it's side, being wedged on handlebars/wingmirrors, having gloves shoved into the cavity, carrying by the chin strap/ chin guard would weaken it over time. I've been riding 7 and a half years now and had 3 helmets haha, one change through crash damage and one because I just found a Shoei I fell in love with