So what exactly causes this? Is it UV? My '03 Pilot lenses look like new, mind you its garaged every day. However, the '05 Civic I had was never garaged, and its lenses looked like new after 5 years. I have never experienced the hazy lenses but have seen plenty.
A variety of things can cause your headlight lenses to become scratched, oxidized, hazed, etc. Acid rain, UV, road debris along with the age and composition of the headlight lens are the main factors. Fortunately, it's very easy to correct.