This section is from the book "Everything About The Dogs", by Alvin George Eberhart. Also available from Amazon: Everything About Dogs.
Many novices will appreciate a few lines upon the subject of how to determine the age of a dog. Fortunately, there is not the amount of roguery in the canine fancy as there is in "horsey" circles, but now and again no doubt the amateur gets "bitten." To a certain extent the age can be determined by examining the teeth. A dog of mature age has forty-two teeth, and these are complete in number at the age of about eight months. This clearly indicates the age so far, but after that there is no method so certain, and a good deal of judgment is needed. As a rule the teeth of an old dog will be dark and lacking in the pearly look of a young animal's teeth - but even this must not be too much relied upon, as cunning dog dealers take care to use a tooth brush freely if such be necessary, and, moreover, dogs' teeth vary a great deal, as ours do.
Generally speaking, the age of a dog can best be adjudged by a combination of appearances. Old dogs are quiet and stolid, whereas youth carries activity with it. Moreover, curious as it may sound, grey hairs will tell a tale in dogs as well as in our own case, for it is often noticeable that a dog has become gray in the face and about the ears. So we have several points to judge by, i. e., appearance of coat as regards greyness; general demeanor; condition of teeth. To these may be added the state of the claws, which in some breeds are curled round and often disposed to be sore and festering. Finally, we may suggest as good advice that the opinion of an expert be taken in any case of doubt. The novice should never purchase a dog at a show or upon any such sudden inspiration without being guided by a competent adviser.
 
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