I would say that sums it up nicely. The plastic itself has UV inhibitors out of the factory. I outgasses over time and leaves the lenses with no protection. I had Lamin-x covers installed after I had the headlights replaced. UV protected-but I'm sure the same thing goes. 1-2 years and the UV will start to degrade those. I have the Lamin-x spray protectant for them with UV blocker. The installer said to shelve it for the next year and to start useing it after thet. Once a month or thorough cleaning.

http://www.lamin-x.com/automotive-s/47.htm

Lamin-x Protective Films - Protect with Style