What they probably did is put some helmets in a heated chamber and "accelerated" the aging, as reaction rates (breakdown) is correlated to various factors, such as temperature. Of course, the storage temperature cannot be raised indefinitely, as other reactions will occur, so this aging may take half a year or more to complete to simulate a 5 year shelf
Since helmet manufacturers are not required to guarantee product life up to X years, it doesn't really make sense for them to pay maintain a chamber to store the necessary samples as well as perform the testing, as it can cost quite a bit. Chances are the 5 year product life comes from leveraging data from other industries, such as those regulated by the FDA.
While the materials are probably good for more than 5 years, plastics may be "stable" for a relatively long time, and then experience sudden degradation in performance. For critical safety equipment, there is usually a big buffer before this happens, but there is no way of telling when it will happen.
My suggestion to you is to stop buying expensive helmets if you feel the need to stretch out their life, and buy cheaper helmets and replace them more often as the UV inhibitors will be consumed more rapidly, especially if you ride as much as you say you do.