This is an interesting piece. It discusses cats, too. Hamsters are middle aged at twelve months!Dogs tend to be fairly rambunctious for two to three years before they settle into a kind of calm, middle-age period...
Around the end of middle age—about 9 years for a medium-sized dog—you’ll need to start watching for various health problems that take place in the senior years: rotting teeth, arthritis, liver and kidney diseases, blindness, deafness and, eventually, a kind of cognitive dysfunction that’s the equivalent of a canine Alzheimer’s disease. Females may also experience urinary incontinence.