Dogs are considered senior from around 7 years old for medium breeds, earlier for giant breeds, and slightly later for small breeds. Senior dogs have changing nutritional needs — they're typically less active and prone to weight gain, their digestive efficiency decreases, and joint health becomes a growing concern. Senior dog food formulas address these changes with reduced calorie density to prevent weight gain, higher fibre content for digestive health, lower phosphorus to support kidney function, and added glucosamine and chondroitin for joint support. Many senior formulas also include elevated antioxidant levels to support aging immune systems. If your senior dog has a specific health condition such as kidney disease, heart disease, or diabetes, consult your vet about whether a prescription therapeutic diet is more appropriate than a standard senior formula.