Most if not all plastic and expanded foam material eventually dies from outgassing plasticizers and other preservative chimicals through the years. Not only during use, but just from sitting around. That's why really old safety equipment are just not that safe to use, even if the look outwardly OK.
I still remember when I put on my San Marco racing ski boots that my brother sent to me from Wisconsin after spending years in storage in his basement. Although it looked perfect when I took them out of the shipping box, the boots literally exploded into many pieces the moment I buckled them on in the not so warm ski lodge....I suspect tha plastic and expanded foam materials in old helmets go through similar degeneration after some time.....