Aging in dogs
Aging in dogs varies from breed to breed, and affects the dog's health and physical ability. As with humans, advanced years often bring changes in a dog's ability to hear, see, and move about easily. Skin condition, appetite, and energy levels often degrade with geriatric age, and medical conditions such as cancer, kidney failure, arthritis, dementia, and joint conditions, and other signs of old age may appear. The aging profile of dogs varies according to their adult size (often determined by their breed): smaller dogs often live over 15–16 years (the max is 20 years ), medium and large size dogs typically 10 to 20 years, and some giant dog breeds such as mastiffs, often only 7 to 8 years. The latter reach maturity at a slightly older age than smaller breeds—giant breeds becoming adult around two years old compared to the norm of around 13–15 months for other breeds. Terminology. The terms "dog years" and "human years" are frequently used when describing the age of a dog. However, there are two diametrically opposed ways in which the terms are defined: However, regardless of which set of terminology is used, the relationship between dog years  and human years is not linear, as the following section explains. Aging profile. They can be summarized into three types: No one formula for dog-to-human age conversion is scientifically agreed on, although within fairly close limits they show great similarities. Researchers suggest that dog age depends on DNA methylation which is an epigenetic process. Epigenetic changes occur nonlinear in dogs compared to human. As a rough approximation, the human equivalent of a one-year-old dog is between about 10 and 15 years—a one-year-old dog or cat has generally reached its full growth and is  sexually mature, although it might still be lanky and need to fill in a more mature musculature, similar to human teenagers. The second year is equivalent to about another 3 to 8 years in terms of physical and mental maturity, and each year thereafter is equivalent to only about 4 or 5 human years. Emotional maturity occurs, as with humans, over an extended period of time and in stages. As in other areas, development of giant breeds is slightly delayed compared to other breeds, and, as with humans, there is a difference between adulthood and full maturity (compare humans age  20 and age 40 for example). In all but large breeds, sociosexual interest arises around 6–9 months, becoming emotionally adult around 15–18 months and fully mature around 3–4 years, although as with humans learning and refinement continue thereafter. According to the UC Davis "Book of Dogs", small-breed dogs (such as small terriers) become geriatric at about 11 years; medium-breed dogs (such as larger spaniels) at 10 years; large-breed dogs (such as German Shepherd Dogs) at 8 years; and giant-breed dogs (such as Great Danes) at 7 years. Life expectancy by breed. Life expectancy usually varies within  a range. For example, a Beagle (average life expectancy 13.3 years) usually lives to around 12–15 years, and a Scottish Terrier (average life expectancy 12 years) usually lives to around 10–16 years. Two of the longest living dogs on record, "Bluey" and "Chilla", were Australian Cattle Dogs. This has prompted a study of the longevity of the Australian Cattle Dog to examine if the breed might have exceptional longevity. The 100-dog survey yielded a mean longevity of 13.41 years with a standard deviation of 2.36 years. The study concluded that while Australian Cattle Dogs are a healthy breed and  do live on average almost a year longer than most dogs of other breeds in the same weight class, record ages such as Bluey's or Chilla's should be regarded as uncharacteristic exceptions rather than as indicators of common exceptional longevity for the entire breed. A random-bred dog (also known as a mongrel or a mutt) has an average life expectancy of 13.2 years in the Western world. Some attempts have been made to determine the causes for breed variation in life expectancy. Sorted by breed or life expectancy. These data are from Michell (1999). The total sample  size for his study was about 3,000 dogs, but the sample size for each breed varied widely. For most breeds, the sample size was low. For a more comprehensive compilation of results of longevity surveys, search for breed specific tables. Factors affecting life expectancy. Apart from breed, several factors influence life expectancy: For more information, see Health effects of neutering. A major study of dog longevity, which considered both natural and other factors affecting life expectancy, concluded that: Effects of aging. In general, dogs age in a manner similar to humans. Their bodies begin to develop problems that are less  common at younger ages, they are more prone to serious or fatal conditions such as cancer, stroke, etc., they become less mobile and may develop joint problems such as arthritis, and in old age often become less physically active and may even develop dementia. Additionally, they become less able to handle change, including wide climatic or temperature variation, and may develop dietary or skin problems or go deaf. In some cases incontinence may develop and breathing difficulties may appear. The most common effects of aging are: Importance of diet in aging. By changing the nutrition of a dog's diet  as it ages, certain ailments and side effects of aging can be prevented or slowed. Some important nutrients and ingredients in senior dog diets include: