The term “senior” is used to describe the older, aging pet. Because of the variability in aging based on species and breed, there is no specific age for senior status. Moreover, the criteria for senior age status differ for dogs and cats. The 2019 AAHA Canine Life Stage Guidelines define senior as “the last 25% of estimated lifespan through end of life.”¹ The 2021 AAHA/AAFP Feline Life Stage Guidelines and the 2021 AAFP Feline Senior Care Guidelines define senior as cats greater than 10 yr of age.^2,3

Additional descriptive terms may be used to describe a senior pet’s condition, such as “frailty,” “immunosenescence,” and “inflammaging.” Frailty is a syndrome seen with advancing age initially described in people, in which the patient has a decreased functional reserve that leads to a decline in physiological and cognitive performance and a greater vulnerability to adverse medical outcomes. ⁴ Scoring systems exist for assessment of frailty in dogs, and increased frailty scores may be a risk factor for death, although more studies are needed.^4,5 Feline phenotype and index-based frailty scales are also available.³

Aging may have multiple biochemical changes that are noticeable only in the later stages of the age-related change. Earlier identification of age-related changes may improve the quality and quantity of life, but additional research is needed to detect those early biochemical changes and be able to use them as screening tests. Immunosenescence is the process of immune dysfunction that occurs with age. Immunosenescence involves the remodeling of lymphoid organs, leading to changes in the immune function of the elderly, which is closely related to the development of infections, autoimmune diseases, and malignant tumors.⁶ Evidence for this phenomenon has been reported in human medicine, but there is limited evidence in veterinary medicine. Inflammaging is another concept reported in human medicine, which posits that the rate of aging and age-related disease may be due to a chronic progressive proinflammatory phenotype. ⁷ The occurrence of immunosenescence and inflammaging in dogs and cats has yet to be validated and should be considered as hypothetical concepts in veterinary medicine until further evidence emerges.