The time when your dog is considered a senior depends largely on his or her breed and measurement. According to Dr. Johnny Hoskins in Geriatrics and Gerontology of the Dog and Cat, small breed dogs (lower than 20 kilos) are of their senior years round 9 to 13 years of age. Medium-sized dogs (21 to 50 kilos) round 9 to 11. 5 years; massive breed dogs (51 to 90 kilos) round 7.5 to 10.5 years and big dogs (greater than 90 kilos) between six and 9 years. In common, smaller breed dogs stay longer.