A German Shepherd pup reaches its full physical size and stops growing at around 2 years of age. However, the dog may not be done with hormonal changes at this age.
Dogs, like humans, go through infancy, childhood, adolescence and, finally, adulthood. German Shepherds don't reach emotional maturity until around 3 years of age. This is when the dog is considered an adult. The time between the second and third years of a German Shepard's life can be challenging for an owner, as this is the time when the dog enters sexual maturity. Behavior problems can emerge during this time, but at 3 years of age, the dog is fully grown mentally and physically.