<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

When is a medium sized dog considered a senior