Older chickens going gray?

[BDutch]

Yes, I considered that to fall under the category of boredom, as in stuck in a small pen without anything to do, but that's worth mentioning on its own as well. Bare minimum 4 square feet per bird inside the coop and 10 square feet per bird outside of the coop.

In general. If you have small bantams, in a mild climate and let the chickens free range daily, you don't need so much space.
If you have a couple of roosters, very active chickens, and big breeds this is probably not enough. And if you have long and cold winters you certainly need more coop space.

 

[Rare]

I have a 5 y/o hen, which have super red feathers without any white or silver parts.

 

[BDutch]

I have a 5 y/o hen, which have super red feathers without any white or silver parts.

5 yo is not old for a normal chicken.
E.g. Laying hybrids are not normal chickens but laying machines that often die very young.