I my opinion part of the reason some dogs grey earlier than others can be due to stress levels.
My dog Toby that lives at my parent’s house started turning grey when she was around 2. She has been hit by a car when she was 10 months old and suffered a severely broken leg requiring surgery. Then a year later tore her ACL and had another surgery.

Buddy who is under consent stress started turning grey when he was 2, only a year after I adopted him.
Sierra who is 4 has just started getting a little grey in her muzzle. Just on one side under her nose.. it looks kind of funny!

Does anyone else think it may have anything to do with stress and/or trama?