Muscle wasting usually starts when a dog becomes a senior. Remember that one human year equals seven dog years. Large dogs become seniors when they are 6-7 years old. Small dogs become seniors in their teens.
Approximately seven to twelve years old, the muscle mass will change. Lower caloric density in the dog's food is recommended, although protein level should remain normal to maintain muscle mass.
Muscle wasting shouldn't necessarily be expected as a normal part of aging. Muscle wasting is often a sign of a serious disease at any age. Please see a vet.
Different breeds age differently. Smaller dogs have longer life spans, while larger breeds age more quickly. It also depends on the overall health of the dog.