For cats and dogs at the age of 1 year, it is the equivalent to the human age of 6-7 years old. Large breed dogs especially, age faster than small and medium breed dogs. So a large breed dog at the age 5, is considered getting on to be a senior.
Noble Paw Veterinary Hospital suggests pet owners bring in their pets for annual physical exams as an excellent preventive measure for potentially life-threatening conditions or disease if caught early. (Yearly exams for pets would be like a human going for a physical exam every 3-4 years!)