The age at which a pet is considered a senior can vary depending on the species and size of the animal. Generally, for dogs and cats, the transition to the senior stage begins around 7 to 9 years of age. However, larger dog breeds may be considered seniors as early as 5 to 6 years of age due to their shorter lifespans and faster aging process. It's important to note that individual animals may age differently based on genetics, overall health, and lifestyle factors, so it's best to consult with a veterinarian who can provide specific guidance based on your pet's unique circumstances.