@ what age do black/tan dogs begin to go gray?

Our daushunds - long haired red and smooth coat black & brown had grey on their muzzle arount 9-10, the black brown passed when she was 16 and by then her face, eyebrows, all brown areas were grey.Tthe long haired red passed when he was 12 1/2. The long haired red was a rescue with questionable nutrition and vet care the first 7 years of his life though and at 12 his face was all white.