I am not a scientist. I ride motorcycles. I wear a helmet, best I can buy (at least most expensive). I like to buy stuff so I get new helmets all the time. As I age and have less disposable income I am sure I will buy fewer new helmets, and will keep helmets for more years. I don't plan to do any statistical studies on all this. I just plan to ride as much as I can. I doubt that a few more years on a helmet will make much difference to me. If I crash hard enough that the age of the lining becomes a factor, I will probably have other things to worry about, like how did I get into Hell and where are the women.
Normal is just a setting on the dryer.
2009 R1200GS A, 2008 K1200GT