Hm. I don't get what such tests would show... burying a helmet or leaving it out in a car in the sun are not intended uses of a helmet. I doubt that would even simulate normal aging of one.

Anyway, I agree with the rule of thumb: when in doubt, replace the helmet.

A while back, I did ask a faculty member in the materials science and engineering department whether degrading EPS was a concern with bike helmets, in particular within 3 years as some helmet manufacturers suggest. He said, under normal use, it was not. That does not mean I will keep the same helmet for 10 years, but I am not rushing to buy a new one, either.