Whenever the graying gene goes off. Sometimes it is age related, other times I think it is stress related.
I bought my husband a 7 year old appaloosa gelding that was sorrel chestnut with a tiny white spotted blanket on his rump. Very tiny blanket. Two years later we moved him to a smaller farm. It was his first time away from what he knew was home, and he freaked out with the move. Unless he was in the company of the older gelding he was a wreak. Within a year the color was completely different: dark chestnut with roaning on the head, dark chestnut on the neck and lower legs, and white completely around from the withers to the tail. You would never have known it was the same horse.
Same thing with my old mare. Every spring when she shed out there would be more grey around her face. I also thought it was funny the first time I found a grey hair in her mane. I guess she was about 20-21 when I found the first one in her mane.