The component in helmets that supposedly degrades is the EPS foam, which is amusing since that is the same stuff we cant seem to get to degrade in landfills. Im sure that it does lose its ability to cushion to some extent, and that is likely due to the environment it has been exposed to. But, when a helmet manufacturer claims their lids need to be replaced in X number of years I become extremely sceptical.
There are far too many environmental variables to make a blanket statement like that, so my gut feeling is that these manufacturers, who have no small conflict of interest in coming up with this replacement schedule, err on the conservative side such that no possible abuse youve given your helmet in 5 years would result in reduced safety. A helmet that was worn a dozen times a year and stored indoors is bound to be in better shape after 5 years than a helmet worn to work every day for one year and left on the handlebar all day.
Wearing any helmet is a conscious decision to improve your safety. Certainly we can make our own decisions about how degraded those 5 year old helmets are.