Dictionaries define 'geriatric' as 'pertaining to old people' and the World Health Organization (1963) has defined 'middle-age' as being 45-59 years, 'elderly' as being 60-74 years and the 'aged' as over 75 years of age. In human terms the elderly should be regarded by society as a useful resource because of their knowledge, skills and experience but the aged most often need assistance. Unfortunately there is no similar classification for our domesticated species and there is no specific definition of a geriatric animal, though we all recognise external signs of increasing age such as greying of the muzzle, stiffness in movement, changes in posture, reduced responsiveness to outside stimuli, and so on.

There are many problems about defining life-stages based on chronological age in cats and dogs because breeds have differing rates of ageing, lead different life-styles and have different life expectancies. I would therefore like to propose a simplified classification scheme (Table 1.1) based upon functionality rather than chronological age which can be applied at any time to any individual.

There are many theories about the ageing process including the concept that all living creatures are genetically programmed to age - a 'biological clock' theory. Most higher living organisms have a relatively brief life consisting of the following basic life stages: conception, growth, reproduction and death. Only a few species (including humans and domesticated pets) pass through a post-reproductive senescent stage known as 'old age'. In the wild, most animals have predators that prevent the frail and infirm from surviving.

The ageing process is complex and we have yet to discover its secret. The person who does - and can find a way to delay it - stands to make a fortune!

In humans, the mean life-expectancy can be predicted based upon sex, race, socio-economic and other factors. For example, in Western society women live longer than men and smokers have a reduced life expectancy. Factors affecting the life expectancy of cats and dogs have not been fully determined and within breed and across-breed comparisons have not been made though it is generally accepted that large and giant breeds of dog have a shorter life expectancy than small breeds.