[header] How to find the cause of aggression in older dogs [title] Detect aggression with unfamiliar people or pets. [step] Older dogs do not always adjust well to change. If you bring unfamiliar people or pets around your older dog, it could become very anxious and display aggression toward that person or pet. 
[substeps] Dogs with dementia (canine cognitive dysfunction) have trouble recognizing familiar faces. If your dog has dementia, it could become aggressive toward familiar people or pets because it can't remember who they are.