I have a wonderful and sweet 2 year old Rott Named Magnus. He is a great dog, but he has issues with strangers. He recently bit a teaching companion of my father's. He did not break skin, but this behavior worries me, as my parents often have "strangers" come to the house. He has growled at many of them, and I am afraid of him biting any children in the neighborhood. Can I get him to stop, or is he too old? He is a shelter dog, so this may be a previous learned behavior.