Aging has been defined as the biological process by which the body changes throughout development, especially as it moves into adulthood. In general, aging is related to a structural degradation that in turn implies losses in functional capacities , with special emphasis on those of adaptation and self-care.

There is currently no consensus in the scientific community on the precise nature and definition of ageing. However, we can distinguish three types of ageing: primary, secondary and tertiary or . Each of these types encompasses different changes and is determined by specific causes.

Types of aging

The main types of ageing are as follows.

1. Primary aging

When we talk about primary ageing we are referring to a series of progressive and inevitable changes that occur in all people as the years go by. As with other types of ageing, it implies a deterioration in general functioning and in the capacity to adapt to the environment.

All non-pathological processes that occur as a result of age are classified as primary ageing; this is why it is also called “normative ageing”. It occurs throughout adult life, although its effects are much more noticeable in advanced ages, especially in people who are not in good health.

Among the changes that make up this type of aging are menopause, weakening and graying of the hair, decrease in the speed of cognitive processing, loss of strength, the progressive appearance of sensory deficits or the deterioration of the sexual response.

The biological processes involved in primary aging alter physical functioning, but are also associated with psychological and social changes. The latter are influenced to a greater extent by context, although when talking about inter-individual variability this type of ageing overlaps with secondary ageing.

Causes of primary aging

The main theories on primary aging conceptualize it as a pre-programmed process at the genetic level . Factors such as the limited capacity for cell regeneration and the progressive deterioration of the immune system have a key influence on this type of ageing.

The theory of genetic programming states that genes that trigger aging are activated with maturity, and that of the pacemaker proposes that these changes are due to the hormonal imbalance produced by the “disconnection” of the biological clock of the hypothalamus. According to the immunological theory, in advanced ages the immune system starts to attack the body.

Other perspectives argue that primary aging is a consequence of the accumulation of damage in the body, and not of immutable genetic factors. These hypotheses, which are generally less accepted than genetic ones, are known as “non-genetic cell theories” or “random damage theories”.

The free radical theory, the most popular in this group, states that the release of free electrons resulting from the normal activity of the organism causes cumulative damage to cell membranes and chromosomes .

Other close hypotheses attribute the deterioration to the spontaneous construction of harmful molecules, to the inability of the body to protect itself indefinitely from harmful environmental influences, to the accumulation of errors in protein synthesis (which would alter gene transcription) or to the normal effects of metabolism.

2. Secondary Aging

This type of aging consists of changes caused by behavioral and environmental factors , unrelated to natural biological processes. It is often said that secondary ageing is that which can be prevented, avoided or reversed, but this is not always the case; the key characteristic is the non-universality of the processes involved.

The main factors determining the intensity of secondary ageing are health status, lifestyle and environmental influences. Thus, suffering from illnesses such as cardiovascular disorders, having an unhealthy diet, being sedentary, consuming tobacco, exposing oneself directly to the sun or breathing polluted air all promote this type of change.

Many physical and psychological deficits typical of old age can be considered a consequence of secondary ageing despite the fact that they tend to be seen as manifestations of the primary one; for example, pathological cognitive impairment and cancer become much more frequent as age advances, but they do not occur in all people.

3. Tertiary Aging

The concept of tertiary ageing refers to the rapid losses that occur shortly before death . Although it affects the organism at all levels, this type of ageing is particularly notable in the cognitive and psychological fields; for example, in the last months or years of life the personality tends to become destabilised.

In 1962 Kleemeier proposed the “terminal drop” hypothesis. This author and some longitudinal research have suggested that, as death approaches, cognitive abilities and adaptive capacity deteriorate very markedly, leading to increased vulnerability.

The Birren and Cunningham cascade aging model proposes that the three types of aging interact with each other, so that their effects are mutually reinforcing. Thus, secondary ageing causes an intensification of the effects of natural biological deterioration, and these changes are even more marked at the end of life.