Doesn't really mean much, but I've got a couple of helmets around that are in the 30-40 year old range (don't use them!) and the foam that is against the shell of the helmet looks and feels fine, though some of the plastic bits of the liners have disintegrated. One old Bell was lined with leather and looks to be wearable last I looked. Also probably doesn't mean squat, but I have cut into old insulated ice boxes on boats and the inside of the foam appears fine and structurally sound, even if the exterior surface is extremely grungy and dinged up. As some have suggested, I don't think the 5-year rule is based on much science. There may be some chance of degradation of certain parts of a helmet, especially if used heavily. For example, how many small drops will a helmet experience in 5 years? However, I am strongly suspicious that the main science on this comes from the sales and marketing departments.
For my part, I travel not to go anywhere, but to go. I travel for travel's sake. The great affair is to move. Robert Louis Stevenson