I found this on the Snell site.
Motorcycle Product News' Bob Jackson sat down with Snell's Gib Brown, director of test development and the foundation's West Coast lab manager, to discuss the criticism. Additionally, Brown offered a history of Snell and detailed its testing procedures.
MPN: There have been various theories over the years that motorcyclists should purchase new helmets every three years or so. Has that been a marketing ploy, or do the materials used in motorcycle helmet construction fatigue over a period of time?
Brown: I think "fatigue" is probably the wrong word to use. What happens is that many manufacturers use glues to put the liner into the shell. We've seen cases where the glues would degrade the liner. We've also seen liners degrade from perspiration and hair oils, and they become compacted through normal use. Snell finally looked at this and said there's no way we can tell an individual how long his or her helmet will be good because the user is the only one who knows how a helmet has been treated. But as a general policy, because of normal degradation and improvements in the helmet, Snell recommends that helmets be replaced every five years.
With this, I think that the 5 year rule of thumb is a catch-all........and just that a "RULE OF THUMB"...not a hard and fast requirement.
Lemmings non sumus
"All the inconvenience and sweat and discomfort of body armor suddenly pales when you're sliding comfortably down the highway on all fours." -ghostdncr