I don't think it's the EPS that's the problem but the glues used to hold it together.

I know with safety boots the manufacturer date stamps them and when I asked they said it wasn't the sole or the leather they were worried about but the glue holding it togther (even when it was also stitched).

I used to have one of those helmets with the fabric cover when I was a kid and I can tell you the polystrene sure did get dented and discolored. The plastic cover that is now glued in place does really help.

Also the straps- think like seat belts that have become discolored- they are not longer safe to do their job. Imagine a strap giving way on your helmet as you speed down a steep hill- it might not hurt you but think about any other cyclist behind.