Cycling glasses life span I've had my Tifosi sport glasses with photochromic lenses for about 4 years. Lately, when I rode them in 10 am morning sun, my eyebrow and eye started to feel terrible pain, that passed once I removed my sunglasses. A friend of mine, that works in a bike shop said that polycarbonate lenses have a life span of about 3 years. I've bought a new pair of glasses, but now I'm trying to decide what to do with the old pair. So the question, do polycarbonate lenses really degrade with wear, time and when wiping them?
Sunglasses have two enemies: dirt and UV rays. 

The polycarbonate itself is fairly tough and can last over ten years. It’s likely to  get scratched up before the UV starts to cause it to yellow or ‘craze.’ The coatings are more delicate. The anti reflective coatings are easily scratched especially if you use your sweat and dirt soaked jersey to 'clean' them.

The photochromic layer itself is activated by UV light but is also damaged by it. Generally, one can expect photochromic lenses to last 1-3 years depending on use. Keeping the sunglasses in a case (and away from UV rays) when not using them helps protect them.