Lily was 2 and I took her to a new vet because old vet refused tests I requested. The new vet took one look at her and said "we need to run a full thyroid panel on her". I asked why, since that was the very test my old vet refused my request......new vet said "she has a greying muzzle already, darkening of the saddle area and seems "puffy". Test came back she was hypothyroid. Once on the meds, her muzzle regained most of her coloring back.
May be just a small indicator of thyroid issues, but having a full panel done now will do two things: 1. see if her levels are low, due to aging. 2. if numbers are "normal", this will give you a baseline for the future - establishing what HER "normal" level is at the present.