Recently Paul and Andrew of Luckygunner.com did a comprehensive ballistic evaluation of various lenses to determine their integrity etc. While this may initially seem to only hold merit for other situations (involving live fire etc.), parts of it are extremely relevant when it comes to paintball goggles. Here's an excerpt:
"Every pair of older eye protection absolutely failed to stop basic “threats” which were stopped by comparable eye protection of newer manufacture or less use.
The highest quality example of the “older” group was the ESS goggle, which passed all military ballistic testing and which I personally wore in Iraq for almost all of 2006. ESSgoggles and glasses were in use by nearly every Marine and Sailor in the area, and I constantly saw how effective they were.
Even so, age – and constant exposure to UV rays from sunlight – takes its toll on polycarbonate eye protection. For this reason, I would avoid using eye protection that is more than a few years old and/or has seen a lot of sunlight. Knowledgeable military sources informed me that the life cycle of military eye pro is expected to be six months."