No matter how good it may look, or how well you take care of it, age still takes its toll. Even with minimal use, a helmet is affected by things like the acids and oils in sweat, haircare products, pollution, exposure to UV rays, etc. At about the five-year mark, helmet interiors begin to show wear and/or deterioration, which should serve as an alert to its overall condition. The helmet's fit may begin to feel a little "loose", not as snug as it once did. This unseen aging and deterioration of the EPS liner and fiberglass shell can affect the helmet's ability to perform in an impact as it was originally designed.
The 5 year rule should be taken from the date of manufacture which is stamped on the inside of a new helmet, or in the caes of an arai on the helmet strap, and not from the date you bought it as retailers buy in bulk they may have had the helmet a year or so before you buy it, just check when your buying it...Apparently Shoei helmets only deteriorate once they start to be used if they are kept out of sunlight in a cool place before being used, i have been informed, but again its up to yourself to check how old the helmet is your buying....