Can't see any point in replacing them.
Mine only exists to keep SWMBO happy and I've conceded that on some (not all) charity and organised rides the insurers insist on them so I'll wear one. It gets worn when the weather's cold, and a few other times so doesn't get much UV exposure, it's been dropped a good few times but hasn't cracked yet. I did have to stick the inside lining back onto the outside harder plastic with No More Nails a couple of years ago. It'll be on my head on Sunday for the Air Ambulance ride because they're now compulsory there.
As I accept the available evidence that it is of no net safety benefit I can see no point in ever changing it. 6 years old and should still be in use in another 6.