Okay I was incorrect it started after 7th year molt
https://www.feathersite.com/Poultry/CGP/Phoen/BRKPhoenix.html
Still could be related in a way to what you have going on but earlier.
So, feathersite actually has a page dedicated to birds that as they age turn more white. Interestingly from my experience my old Barred/Cuckoo (unsure which one he was expressing) as he aged his hackles became more cream less barred. Passed around 9-10 years old. My 10 year old kraienkoppe cross...