Just keep in mind that the five year recommendation is made by folks who want to sell helmets. I've never seen any study which specifically looked at the deteoriation of helmet materials with time and storage conditions so we're all flying blind here with no data.
To my mind, a blanket 5-year helmet life is like a blanket one-size-fits-all expiration date for photograhic film - in both cases, it's storage conditions, not time that is critical. In proper storage conditions and with light/careful use, I would expect a helmet to offer all (or nearly all) it's protective properties for a lot longer than five years. Likewise, a helmet that sees a lot of UV and ozone exposure in hot conditions, might be trashed in less than a year.
There is nothing very magic or mysterious about helmets and the key thing is that the other shell hasn't seen an impact that can crack it and that the EPS foam inside hasn't been compressed by impacts (or storage on a mirror stalk) and isn't deteoriating. If you're reasonably sure the helmet is in good condition, I say ride on.
- Mark