I agree with everything you wrote except the first sentence. Based on the study, it would seem that material aging and UV probably are not a factor They didn't control for UV exposure, but the large data set would suggest that they would have captured at least some "UV damaged" helmets. Is there any data from manufacturers to support the recommended replacement interval?

I think that helmet manufacturers recommend replacement to sell more helmets, using safety as an issue.

All of this points out that time is not a factor. So why replace a helmet after a certain time period at all?

I think this points out that a good "pre-flight check" should include a helmet inspection, along with air pressure in your tires, brakes working, quick releases secured, etc. I've banged a helmet twice in 40 years of riding. Each time I replaced it.

One thing that isn't mentioned is that the fitting systems seem to degrade over time. I replace my helmets when the nylon webbing starts to get gunky and start to slip, making it more and more difficult to put it on and adjusted correctly. Most thelmets nowadays have a plastic thing in the back of your head. I had one on an older helmet snag and break awhile back. I have two helmets, I rotate them so they dry out. I find that the pads last longer when given a chance to dry, which is part of the fitting system.

Let's say you take a helmet into a shop, like I did a few years ago just to see what they would say. They didn't really inspect it, but they did ask how old it was. They recommended I replace it becuase it was over 5 years old. Why? Manufacturer's recommendation-the same guys that want to sell you a new helmet. And it was the right thing to do at the shop. To say otherwise might lead to a future lawsuit. I wonder if they would have inspected it if I said it was 2 years old. I'll try that experiment some other time.