A baby dog is referred to as a puppy, pup or whelp, notes the Oxford Dictionaries. Most people refer to a dog as a "puppy, pup or whelp" from the moment it is born until it is somewhere between 6 and 18 months old.
After this puppy stage, the dog enters into adolescence. The adolescence stage also occurs between 6 and 18 months old. From this stage, the dog becomes an adult, which is generally considered to take place when the dog is between 1 and 3 years old. Senior dogs are dogs that are 6 to 10 years old. Each dog breed tends to follow its own patterns of development.