How can you noticed how old your puppy?

Veterinarians can give a reasonable estimate of age based upon the teeth, the development of eyes/ears/hair coat, and the puppy's ability to walk and navigate around objects. However, this is not exact and becomes more difficult as the puppy ages; by about 1 year of age a veterinarian can generally give you a time frame within a couple of months, but by 3 years the best a veterinarian will be able to give you is "adult dog, older than 2 years".