At first glance, the answer to this question seems obvious. Surely a puppy is considered fully grown when it has reached its adult stature. But is this actually true? By fully grown are we only talking about physical growth, or is there an element of psychological maturity involved as well?
Since dog breeders first realized that breeding dogs for the companion animal trade could not only be as lucrative as breeding them for work and as hunting assistants, but far more so, determining when a puppy matures into an adult has become considerably more difficult. People requiring dogs to assist them in their physical endeavors want serious, essentially adult dogs. Those seeking companions desire puppy-hood to last as long as possible, at least in the main.
Neotenization is the term used to mean the carry over of traits and behaviors from young animals into older members of the breed or species. Dog breeders have been selecting for this in companion dog breeds since the concept first arose, hundreds if not thousands of years ago; more recently it has become far more significant in the breeding programs of all dog breeds. Many working dog breeds, as technology has advanced, have moved into the companion animal bracket. Or at least more members of most such breeds now live as companion animals than do as working dogs.
With this change in breeding trends, can we still say that a puppy is now an adult when it has reached its full size? With wild canine species there is a clear distinction. Wolf cubs play, adult wolves do not. The same cannot be said for domestic dog species, although the distinction tends to be clearer with those breeds closer to their wild antecedents (forefathers).
If we consider it on this basis, when does a dog mature? Do we hold to the precept that a dog moves from the category of puppy or juvenile to adult when it reaches the height and weight typifying the completion of physical growth for its breed? Or do we wait on psychological maturity? Perhaps our beloved puppy only becomes an adult when he or she switches from initiating play with us, to accepting it when we initiate it with him or her.
If we are only considering physical growth, then with most dog breeds, our puppies are usually fully grown by around two to three years of age. Although their bodies are likely to start thickening when they hit middle-age, just like ours unfortunately have the tendency to do. But if we consider behavioral traits as well, then we might want to add several years on to that, especially for dog breeds that have been companions to us for many centuries.