Is there real science behind helmets turning into pumpkins at ~5 years, or is that marketed obsolescence ideas so companies stay in business?

I totally understand the one-good-smack-and-you're-done aspect, but not the shelf life.

I'm surprised at the surge of non-helmet users here, and pleasantly surprised at the civility of co-existence of alternate viewpoints here. Vive le diference is an e-rarity in these forums. Though of course those of you who don't do as I do are destined to burn in hell for eternity, at least we can be nice about it for your short time amongst the living.