[throttlemeister]

Helmets wear under the influence of UV light and chemicals (external shell), sweat (liner and inner shell), age (all, but liner is the first to go) and use (liner). If not used much, kept clean and stored properly, they can last well past 5 years. Conversely, sweaty, dirty helmets that are used daily for extended periods of time can be ready for the bin in two years.

For most people, 4 to 5 years is recommended. I don't know how it is in the US, but here there is no legal rule for the age of a helmet. As long as it has ECE05 approval, legally it doesn't matter if it is 15 years old. Also, warranty says nothing about lifespan. Warranty says something about how confident a manufacturer is about its product and how it feels how long it should last minimally. If warranty would be taken as guide as to long a product lasts, Apple users would have to replace their laptops every year as it would fall apart after that.

Personally, I rarely use a helmet more than 2-3 years. Not because it is necessary, but because by thenI fancy a change/newer model/whatever.