Gosh, I've never given it one minute of thought and have never seen any sign of degradation in any helmet I - or any of my family members - have ever owned. I usually give them up when (a) I just feel like buying something shiny and new or (b) when the straps are just worn out or too disgusting to wash well enough. A neighbor's dog chewed on one once and I got rid of it a couple years later.

My guess - and I'd like to see actual testing and evidence to the contrary - that is the self serving helmet industry that sets these so-called standards for replacement, just like they somehow duped the public into thinking that cycling is a dangerous sport and requires routine and constant head protection. Bah.