Originally Posted by JohnJ80

Helmet manufacturers recommend replacement after a time period due to material aging, UV, and experience with their products from field evaluations. Every time your helmet takes a hit - thing like dropping it on a hard surface - you cause compressive damage to the material of the helmet that can cumulatively lessen it's ability to provide protection.

Qualitatively, as a ski patroller, when I look at my helmet after three or so years of use, it's got dings all over it from my handling of it and from getting normal bumps in the head in the course of it's skiing usage. Same goes for my bike helmets. We use them a lot and they are not babied but used in real life. The material in them is designed to crush on impact and it does not spring back, hence the damage is cumulative...[SKIP]
...and because we're seeking the maximum effective protection (why wear one otherwise?). Materials age and we don't necessarily take care of things in an ideal manner. Our helmets are heavily used.