According to most veterinarians, a dog falls into the “senior” category around age seven. The size of a dog, however, affects this categorization. Smaller dogs mature slower and become seniors later in life than larger dogs. Animal shelters are filled with healthy and active senior dogs that are in need of a home. When you’re thinking about adopting a dog from your local shelter, don’t look past the older dogs. They make great pets for a number of reasons.