Again, it's because the org doesn't know when you purchased it. To make it easy on them and probably to help with insurance they cut it at 5 from the sticker.
It all depends on how it is stored. Temp, uv and other factors degrade the helmet. Almost every race org has a 5 year limit from the date on the helmet as there is really no other way of knowing when it was purchased.
I can't complain about the pirelli wear, I got a shit load of races and trackdays out of mine. I would say they are on par with the michelins for wear, I have never ran the dunlops.