If there are no physical or mental deformities that prevent the dog from conceptualizing (ie dementia), a dog of any age can participate in dog psychology training and behavior modification training. I often customize the process to suit the dog in extenuating circumstances. The oldest dog I have worked with is 14 and enjoying his lazy days but with a few adjustments to the program, he responded beautifully to the training. Furthermore, I worked with a dog that had suffered a terrible accident, which left him with brain damage. His dedicated guardians were diligent in following my customized training regime and all are living quite happily today. This goes to prove, once again, how deeply instincts are rooted.