4.2 Definitions of ageing
The term ‘ageing’ carries a number of different meanings. It encompasses changes that occur at many levels, from the population down to the molecular. Even at a single level, ageing does not represent a single process, but many processes, which may operate independently. Therefore, the challenge of defining ageing is more complex than it might first appear. At the level of a population, ageing is seen as the progressive increase in mortality with age, often called a ‘mortality pattern’. In population studies, emphasis is thus on the probability of dying, as a function of age, and not on the individual aspects of ageing, as reflected in the changes of a variety of body functions. In studies of ageing at the level of the individual, as in this course, the emphasis is on the patterns of changes that occur during adulthood as the individual matures and grows old.
Ageing is one of the great biological mysteries. Some humans (and certain species of animals and plants) live for decades, whereas their close relatives may have much shorter lifespans. Why is this, and why does ageing occur at all? Many theories to explain the phenomenon/process of ageing have been proposed. These theories fall into two main categories; evolutionary and mechanistic, attempting to explain why ageing has evolved and how ageing occurs, respectively. Although fascinating, consideration of evolutionary theories of ageing is beyond the scope of this course; here we focus on the mechanisms of neural ageing, at the system, cellular and molecular levels, and how age-related changes impact on the functioning of the brain and the quality of life of older individuals. The ageing of an individual is manifest by changes in the tissues and organs, which are made up of cells. Therefore, we need to study ageing at the cellular level, which in turn will bring us to the level of the genes, and of the molecules of which cells are made. It is crucial to recognise and take into account the interactions of genes and gene products with the environment, in the ageing process. Therefore, we must consider ageing as a complex phenomenon that reflects the influences of genetics, the physical, chemical and social environment, and also individual behaviour.