Although there are commonly used definitions of "old age", there is no general agreement on the age at which a person becomes old. Most developed world countries have fixed the age of 65 years as a definition of "elderly" or older persons. Conventionally, the subgroups "young old" (65-74 years), "middle old" (75-84 years), and "old old" (>85 years) have been derived (3). This chronological concept obviously does not reflect the health situation in poorly developed countries, for example, in Africa, where the life expectation is quite short. Moreover, in some countries the date of birth is not even known because of lack of birth registers, and, thereby, the age of an individual has to be estimated. The chronological categorization does not reflect the continuous increase of life expectation achieved in the most recent years and most probably continuing in the near future.
While the chronological definition is somewhat arbitrary, the old age is often associated with the retirement from job or profession for an average individual receiving pension benefits. People aged between 18 and 64 years are regarded as the working population, and both the pediatric and geriatric population are distinguished from that age group. Nevertheless there are many people still working beyond 65 years, often in key roles as senior manager or consultant. At the moment, there is no accepted international numerical criterion for this age, but the United Nations agreed a cutoff of 60 years to refer to the older population as per the definition of an older or elderly person given by the World Health Organization. Again, this classification is mainly attributed to the developed countries in the world where the job or profession determines the personal income and the social status. In contrast, in many parts of the developing world time has little or no importance for the type of income of an individual (4). Elderly often own the highest social status in the community like tribal elders or medicine men.
From a medicinal point of view the aging process, however, is a dynamic process that is mostly beyond human control. The human body undergoes various changes and develops different illnesses causing reduced capabilities. Many geriatric patients suffer from various diseases with multiple-drug treatment. Some of these patients are addicted to alcohol, nicotine, and pharmaceuticals. Reduced kidney and liver functions as well as dehydration may dramatically alter the pharmacokinetic (PK) parameters. Limited audiovisual and ergonomic abilities must be considered. In rare diseases like progeria (Hutchinson-Gilford syndrome) a child may look, feel, and be realized as an elderly person. Considering all the different factors, it is very difficult to define a general cutoff for aging (Table 2). The "Clinical Assessment Scales for the Elderly (CASE™)" have been developed mainly for typical mental disorders in the geriatric population like cognitive competence, fear of aging, obsessive compulsiveness, and others. The test allows assessing patients from 55 to 90 years. In the developing countries, patients from 50 years and more are often recruited for clinical studies in the geriatric population (5). An interesting approach is to use a "functional age" that is independent from chronological and socioeconomic scales (6,7), but so far no functional age scale has been adopted as a general approach (8). The variables in the different diseases are too complex for a single scale.
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