The defensive drive can begin to show as young as 4 or 5 months of age in the form of barking at strange circumstances . It does not develop to the full extent until the dog is 18 to 24 months old and with some dogs not until they are 3 years old. Experienced helpers can begin to introduce a young dog to defense when it reaches puberty, this is usually around 11 to 14 months. New trainers that make the mistake of introducing defense before a dog is mentally mature enough to deal with the pressure are making the biggest mistake of their training career. Pushing a dog into defense before it is mentally mature enough to deal with the stress is the quickest way in the world to end a dog's protection career.
Some dogs have excellent prey drive - but lack defense.
A common example is the black lab that loves to chase balls but could never be trained in protection - because it lacks defense. Unlike prey drive, the defense does not diminish as the dog gets tired. Another way to look at it is that no matter how tired your dog is it is still going to react to someone that is threatening him. Working defensive drive takes a skilled helper that is adept at reading and understanding temperament and knowing exactly how fare a dog can be pressured or threatened in defense before it is pushed into avoidance. Initially, in prey work, the helper is a friend that plays tug or a person that is always trying to steal the prey.