W/r/t to expiration: even if no date I'm sure it's affected by ozone, fuel fumes, UV rays, temp changes - but may not be detectable.
If originally bright white, tan color may indicate chem change. Keep a scrap section inside a zip-loc bag in the dark for a color comparison. Like roof paint: if the top is lighter than the door, it changed.
"it does feel pretty hard but it spreads the impact load over the surface without bottoming out"
True. However: it's a "one-use" device. If you wreck, and not positive that nothing touched it, replace it. Helmets are a throwaway.
I'm sure no one wants to take it off after the season, I suggest Saran wrap.