Originally Posted by
Greenhil
Might have been a sales pitch, but a guy at an LBS told me the materials degrade over time, making the helmet unsafe, or at least less safe. When I bought a new helmet he told me to go home and take a hammer to my old one so no one could use it (it was 20 years old - so it was an extreme example).
Child car seats are another example. Plastics degrade in the sun, though car seats see a lot more sun than a helmet. CPSC recommends a 5-10 year life, or following manufacturer guidelines.
If a helmet's crashed, cut the straps so it can't be used after you throw it away.