Any kind of light isn't a good thing.
Light has nothing to do with a cured tire. The moisture in standard air compressor is the only culprit. I built tires at Bridgestone/Firestone. An unmounted tire is preferable, if mounted stack flat.
After 5 years, it becomes more of a legal issue if one fails going down the highway but most tires do not fail until they are much older. The oldest tire that failed on me was 20 but have had ones that were only 3 that shredded themselves in regular service on a car or SUV that never had any loading other than the vehicle itself and that was a BFG. In my experiences, General Tires have been the worst offender with 3 failures with two doing damage to a car. The 3rd one I got pissed and filed a claim and they actually paid me for damages. I refuse to run any General tire on any vehicle.
Even indirect sun light works on them....
Imo, 100% nitrogen is better than 70%.....but nitrogen is better than plain oxy/nitro for keeping psi constant too. Other than that, keep them covered up and in the dark. Can't hurt.