The aging process refers to the natural series of life stages, beginning with conception and continuing through development, adulthood and finally senescence (geriatric stage). Although it is often misconstrued as such, aging is not a pathological process, but rather comprises the normal time-dependent changes that occur during the life of every individual. As many pet owners who cherish their senior dog or cat will attest, living with a healthy older dog or cat can be a wonderful experience. For example, older pets are often well behaved, are enjoyable to live with, and have a well-established set of endearing habits. Other changes associated with older pets are neither positive nor negative, such as the development of a graying muzzle or a slightly reduced activity level. Undesirable age-related changes are those associated with illness, changes in mobility, changes in cognitive function and the development of behavior problems. The deteriorative changes that may negatively affect a senior pet’s overall health and quality of life are referred to as senescence. ³

A variety of theories have been proposed to explain the phenomena of aging and senescence. ⁴ Several of these focus on genetic controls, such as the somatic mutation and gene regulation theories. Others examine the impact of temporal changes in homeostatic mechanisms of different body systems or the accumulation of the damaging products of cellular aging. For example, the oxidative stress theory postulates that aging is linked to energy expenditure and the cumulative cellular damage caused by the free radicals that are the byproducts of oxidative metabolism. ^{5. and 6.} None of these theories singularly explains all of the changes that are observed during aging; nor are they necessarily mutually exclusive. Most likely, aging and senescence are multifactorial processes influenced by genetics and a myriad of internal and external environmental factors.

From an evolutionary perspective, domesticated dogs and cats present a unique example of two species that enjoy an extended life stage beyond successful maturation and reproduction. Natural selection works principally on survival and fitness up to and throughout an individual’s reproductive age and this impact diminishes as an animal enters its postreproductive years. In the wild, nondomesticated species typically do not enjoy a long postreproductive life, and older animals rapidly experience increased vulnerability to predation, accidents, and disease. In contrast, companion dogs and cats are protected from the pressures that have historically impacted their progenitor species. Many live well into their geriatric years, a life phase that is accompanied by age-related degenerative diseases and increased incidence of various types of cancer.

Because genes that regulate late-life deterioration have not been subjected to the same pressures of natural selection as those that support reproductive success, it can be argued that seeing an increase in the diseases of senescence is a natural byproduct of domestication. Indeed, several of the theories of aging suggest that the same genes that confer early maturation and a high rate of reproductive success may, at the same time, contribute to illness or disease later in life. 7. ^{and 8.} Such genes would persist in a population because of their beneficial impact upon reproductive fitness. In the case of species that invariably die young because of extrinsic (environmental) factors, the negative effects of such genes upon senior fitness would be irrelevant and therefore would be maintained. Conversely, when extrinsic factors are naturally absent or are controlled by human intervention (i.e., the species enjoys a relatively low postreproductive mortality rate), genes that promote longevity may begin to confer a selective advantage. Although the relationship between extrinsic mortality and longevity has been shown to be complex and influenced by other factors, this theory helps to explain the influence of domestication on dog and cat longevity.

The maximum lifespan of the dog is estimated to be about 27 years, but few dogs live to be older than 18 years of age. When all breeds are considered, the average lifespan of the domestic dog is approximately 13 years. ⁹ Intraspecific variations in longevity are of special interest with dogs because of the wide range in body size and conformation between breeds. Many of the large and giant breeds have a significantly shorter lifespan than the average of 13 years, while small and toy breeds tend to live longer. Numerous studies have shown that when examined across breeds, there is a significant negative relationship between lifespan and mature body size in dogs. ^{10.11. and 12.} The shortened lifespan of giant breeds appears to be a result of the artificial selection for extremely large dogs and accompanying rapid early growth rates. Developmental skeletal diseases related to higher growth rates are an important (but not the only) contributing factor to the shorter lifespan of these dogs. For example, a recent pilot study found that mature body weight was more strongly correlated (negatively) with lifespan in dogs than other measures of size, such as height at the withers. ¹³ Although this study did not find breed to be a significant factor, they reported that some heavy but short-in-stature breeds, such as the Bulldog, experienced a shorter lifespan due to the influence of inherited structural disorders. The construction of breed standards that enforce a closed studbook and require absolute reproductive isolation has led to inbreeding practices and the overuse of a small number of individuals (usually males) in an already limited gene pool. This has contributed to the propagation of the more than 500 identified inherited diseases of purebred dogs, many of which may contribute to a shortened lifespan for the affected breed. ^{14. and 15.} Although purebred breeding is less widespread in the domestic cat, inbreeding in purebred cats may eventually lead to similar negative effects upon health and longevity. (Recognizing these problems, reputable breeders use careful selection and screening practices to reduce the incidence of genetically influenced health problems in their breed.)

Because breeds of dogs differ in average lifespan and susceptibility to disease, an individual dog’s breed, size, and current health must be considered when determining whether or not he/she should be considered geriatric. General guidelines divide dogs into four categories based upon their adult size, with smaller dogs considered elderly at a later age than larger dogs (Table 25-1). ¹⁶ However, the dog’s body condition, activity level, overall health, and the presence of chronic or debilitating disease must always be considered. ¹⁷ In general, cats age more slowly than most dogs and do not tend to show breed differences in aging or longevity. The average lifespan of cats that are kept indoors is approximately 14 years, and the cat’s maximum lifespan may be as high as 25 to 35 years. Healthy cats are considered to be geriatric when they are approximately 10 to 12 years old.

In all animals, the biological effects of aging include a gradual decline in the functional capacity of organs, beginning shortly after the animal has reached maturity. A set of preliminary studies reported age-related changes in laboratory values for the dog and cat (BOX 25-1 and BOX 25-2). ^{18. and 19.} Although the limited number of animals that were examined preclude using these data to make generalized conclusions about age-related changes to blood chemistry and cell data, these data suggest that multiple physiological systems are affected during the normal aging process. However, the clinical significance of these changes is not known and may not be relevant in many cases. In addition, different systems of the body age at different rates, and the degree of compromised function that must occur before clinical signs are seen depends on many factors in a pet’s life. Although one pet may exhibit severe pathological effects of aging by 7 years, another may exhibit no clinical signs even at 12 years. It is also not unusual for more than one chronic disease to be present in a single geriatric pet. This variability necessitates that older animals be assessed as individuals, using functional changes in body systems rather than chronological age to categorize them with the geriatric population.