The study of gerontology covers a range of areas including genetics, molecular biology, cellular biology, psychology, and the study of each of the body's vital organs.

Toll of time

The average 90-year-old's brain weighs 10% less than a 20-30-year-old An average person in their 90s has only half the lung function of someone in their 30s Renal function is normally reduced by 40% in later life

Merck's Manual of Geriatrics says that no single theory can possibly explain all the ageing processes - and that intensive research support into the subject has only been available for about 15 years.

As people grow older, certain changes take place in their abilities, behaviour and physiology.

Some theories indicate that these processes are the result of a "programme" run by the body's genes having reached its natural conclusion.

Wear and tear

Others still point to wear and tear - the possibility that the body's cells simply stop dividing and vital organs lose efficiency until they can no longer work.

After the age of 30, hormone levels begin to subside in human beings, the immune system becomes less effective and muscles shrink. Hair falls out, teeth get loose in their sockets, skin sags and wrinkles and joints become less mobile.

Brain mass also decreases as a person gets older. Once the cells of the central nervous system die, they do not get replaced.

So, from the age of 20-30 until the age of 90, the weight of the average brain decreases by 10%.

The hospital where tests were carried out on Augusto Pinochet

In the case of the former Chilean dictator Augusto Pinochet, ageing and ill health have allegedly manifested themselves in the form of a series of small strokes, a heart condition and depression.

The Home Office has now said that General Pinochet is unfit to stand trial for human rights abuses after he underwent medical tests earlier this month.

It would not be unusual for the 84-year-old's cardiovascular system to be impaired through thickened arteries, meaning that additional bursts of adrenalin common in stressful situations - such as a court room - might have a more profound impact than on a younger person.

Intelligence maintained into 80s

Merck's Manual states that mental capacity and intellectual performance, in people not suffering from brain disease, tends to be maintained at least in the 80s - even if task performing around that age becomes more laborious, indicating a slowing in central processing.

Elderly people are less tolerant to alcohol

A fall in ratio of body water to fat means there's less water for alcohol to be diluted in A decrease in hepatic bloodflow means that the liver sustains more damage Inefficiency of liver enzymes means that alcohol is not broken down as efficiently Alcohol has a faster effect on the brains of elderly people

Learning becomes more difficult as people get older, especially of languages - and forgetfulness also increases with age.

But it also notes that stress from either medical illness of psychological factors "can produce delirium and depression can produce a dementia-like syndrome" in the elderly.

Studies in the USA have also indicated that anxiety states are common in later life, with up to 5% of community-based older adults experiencing symptoms of generalised anxiety disorder.

The reasons why these and other factors of the ageing process occur are subject to debate and research.

Free radicals in the air are held by some researchers to be the culprits for cellular deterioration, while some branches of genetics are devoted to researching the likelihood that genes are pre-programmed with a maximum life span.

Many theories

Gerontologists themselves concede that most theories are partly true.

The effects of ageing can be categorised as usual, or average ageing and successful, or pure ageing.

The former refers to the compounded effects of time on the body with disease and damage from environment and lifestyle.

Pure ageing refers to the changes which occur in the body due solely to the effects of time.

It is known that age increases the body's vulnerability to disease, but gerontologists and geriatric practitioners have to be sensitive to the different ways that illnesses present themselves in older people - and the fact that external influences mean that the effects of ageing kick in at different times with different people.

A spokeswoman for Age Concern said: "You can have a perfectly healthy, sprightly 90-year-old, but a seriously disabled 60-year-old. It is not possible to say that at a certain age a person will have certain faculties or standard of health."