Yes, UV and ozone do break down the styrofoam, but unless it's stored in the sun next to an electric motor, it gets most of its damage from the oils and acids it's exposed to when it touches your hair and skin or gets sweated on.
A NOS helmet that had been on the shelf for 7 or 10 years, no, I absolutely would not wear. But if I buy one that's reasonably new, I feel perfectly safe wearing it for five years.
The only reason that manufacturers that have a pro rata trade-in program use the manufacture date, is it's the only way to verify the age of the helmet. They couldn't just take the word of the owner as to when it had been placed in service.
Last edited by OakLeaf; 10-03-2008 at 05:33 PM.
Speed comes from what you put behind you. - Judi Ketteler