I was reading VeloNews (this column) and I found this:

"Nick,
I’ve heard that helmets deteriorate over four or five years, even unused ones. True of false? -Phil
[...]
Helmets do deteriorate over time and especially when exposed to UV. Nic Sims at Specialized told me Specialized destroys any helmets in its warehouse that are four years old or older. So, Phil, you’re better off looking for a new lid."