Why is a senior diet important? When dogs reach senior age, their metabolisms slow down, as do their activity levels. Nutrition is of significant importance at this stage and plays a vital role in maintaining the health, longevity and quality of life of your companion. At what age is my dog considered senior? The larger the breed, the earlier you should switch to a senior diet. For large breed dogs (35kg+), this can be from as early as 6 years of age. For small (up to 10kg) and medium (10kg to 35kg) breeds this is from 7 to 8 years of age.