UV degradation is a non-issue, and will have little effect on a bicycle helmet. The main component of bicycle helmets is simple styrofoam, which, as we know, can endure years of sun on a beach before begins to disintegrate. Most helmets already feature treatments to filter out UV effects, Specialized has done so since the 80's. The main cause of deterioration is chemical degradation, as the solvents and such used in the construction evaporate or break down over time, this is accelerated when exposure to sweat, sunscreen, and such.
I recently bought a new, old-stock Briko Twinnter helmet from 1998 or so, and the materials and structure are in perfect order. I wouldn't worry about wearing it today.