Join Date: May 2011
Location: Northern California
You can usually get a clue by the teeth. If they are sharp, clean and white, with no wear or tartar buildup, you're probably looking at a young dog under 2. If there is tartar present, the dog is probably at least 2-3 years old. If there is significant tartar, wear, or discoloration, probably 5 or more.
You can't really tell by greying on the muzzle because some dogs go grey as young as 2. And I have seen dogs aged 10+ with good hard muscle tone, and dogs as young as 2-3 that were as soft as a stuffed animal. Muscle tone has more to do with exercise and activity level than age.