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."
Every goggle I've bought has come with a set of safety instructions saying pretty much the same thing, but this definitely goes to show that it's not just a CYA (covering your assets) situation for manufacturers, and is in fact something paintballers should probably take into consideration. I believe manufacturers recommend getting new lenses every year, so if you want to have piece of mind that might be something to consider (that being said I know I'm definitely guilty of not buying lenses anywhere near that often). I'm not trying to sensationalize or play nanny, but it's certainly surprising that after only a few years the integrity of polycarbonate lenses can be compromised so much (if the ballistic goggles tested no longer protected against what they were supposed to after a period of a few years, it's likely that your paintball goggles won't either eventually).