To those claiming that UV degradation isn't an issue, can you post some Scientific references please.
UV degradation isn't going to make a helmet fall apart in your hands. Sure, yet lightweight bicycle helmet design is pretty much on the edge of material capabilities from the get go. It won't take all that much degradation before the helmet fails to provide maximum performance is a crash.
Now for the record I'm not saying that you have to change helmets every 2 or 3 years, yet they certainly don't last forever and your risks increase as the helmet ages.