The age at which a dog is considered a senior can vary depending on the breed and size of the dog. Generally, smaller dogs age more slowly than larger breeds, and they can be considered senior dogs around 10-12 years old. Medium-sized dogs are considered senior dogs around 8-9 years old, while large and giant breeds are considered senior dogs around 6-7 years old. However, there are always exceptions to the rule, and some small breeds, such as Cavalier King Charles Spaniels, have a shorter lifespan and are considered senior at about 8 years old. Its important to note that there is no definitive answer to the question of when a dog is considered a senior, but in general, the senior life stage is the last quarter to one-third of a dogs expected lifespan.