So far nobody has dug up a single data point indicating there is any degradation in the protective ability of old helmets. The only issues are related to the liner which is related to comfort, not protection. I would assume that at some time in the past someone did some kind of test, but the details are lost in history. It was probably some test of army helmets during the Crimean war and the 5 year number lives on through the centuries
From a website talking about Expanded Polystyrene: Ageing resistance
All of the properties listed above are retained over the whole of the
materialís life and will last as long as the building itself. EPS is not
altered by external agents such as fungi or parasites as they find no
nutritional value in the material.
Another quote about EPS: Core specimens of EPS removed from freezer walls in place for twenty years have demonstrated no deterioration in the structural integrity or physical properties.