[on2wheels52]

Sidney, I have it that the shell of a helmet breaks down over time. This from the Shoei site, it does read a bit strange, perhaps not a great translation:

"How many years can the helmet be used?
We recommend exchanging in three years.
The purpose of the helmet is to be superannuated, and not to be able to maintain new time and the same performance by a change with the lapse of time like deterioration etc.
The performance deterioration of the helmet progresses chiefly by the following two factors.
AAA) Wear-out of interiors by use.
BBB) Deterioration of plastics component by sunray, especially by ultraviolet rays.
Therefore, after it manufactures it, the product that well, is hardly causes a change with the lapse of time in the warehouse as for keeping.
The influence by ultraviolet rays occurs FRP and, as a matter of fact, the performance decrease doesn't occur few for about three years. However, an interior made of the form of the urethane and an energy-absorbing liner made of the styrene foam might cause wear-out and deterioration because of use. The mind distribution recommends the use within three years too much in a usage commonsense this though it is not needed.
The helmet used for three years or more might not demonstrate an enough protection performance during the accident. Please note it enough.
Neither the helmet that receives a big impact nor the helmet that is damaged and remodeled might demonstrate an enough protection performance during the accident even if it is within in three years. Please note it enough."

I get asked at the pawn shop if I buy/sell used helmets. I tell them heads are too valuable for a used helmet.
Jim