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The 'Wear and Tear' theory of aging was introduced by Dr. August Weismann, a German biologist, in the year 1882.
Aging is the natural process of maturing, or growing, and after a certain period of time, it leads to the death of the person. As time passes, we all age, and eventually die. That is the cycle of life. But why do we age? What causes our health to deteriorate? There are various theories that revolve with the psychology of aging.

Due to the efflux of time and wear and tear, the value of any asset decreases. However, can the same principle be applied to the human body? Does our human body suffer, too and age due to wear and tear? According to this theory, it is due to the external factors that our body is subjected to damage and degradation, hence, our health depreciates. After the wear and tear causes too much damage to our body to sustain, death occurs. With age, the body also loses its ability to damage repairs and combat diseases. Our body is subject to abuse by external elements and also by usage.
Definition
There are various theories of aging, out of which, the 'Wear and Tear' theory of aging suggests that, human beings are simply like assets, and the cells of our bodies are subject to wear and tear. There are various external agents, such as chemicals, ultra-violet rays, harsh climate, etc., that damage our body cells. In addendum, there can be various other reasons, such as smoking and over consumption of alcohol, that can add to the damage done to the body. Lifestyle and over usage may accelerate the frequency of damage to the body.

In short, this theory suggests that aging in humans is due to the degradation which the body is subject to. Let's see some examples to elaborate this biological theory of aging.
Examples
Skin
Skin
The skin is the most delicate and exposed part of the human body. Apart from weather, sunlight, bruises, pregnancy marks, etc., all are the result of wear and tear the skin is being subjected to. With age, there is thinning of the epidermis, and it appears more wrinkled.
Bones
Bones undergo degradation too, and with age, there are many who complain of osteoporosis (weakening of the bones) and osteoarthritis (pain and stiffness in the joints). With efflux of time, our muscles tend to contract, thereby losing mass. Further, we suffer from joint pains due to increase in tensions. Of course, these issues can be combated with regular exercise.
Eyesight
Eyesight
With age, we may tend to suffer from loss of vision to some extent, or fall prey to diseases like cataracts. Our eyes lose the natural ability to develop tears, and that leads to dry eyes. The occurrence of 'presbyopia' becomes common, which makes it difficult for the patient to read fine print. These symptoms are usually observed after the age of forty.
Hearing Loss
Hearing loss can also be a result of aging. Of course, exposure to loud music, smoking, etc., may serve as an addition to the hearing loss. Prevention of age-related hearing loss is extremely difficult, and it might be a result of damage to sensitive hair cells inside the cochlea.
Impact on Blood Vessels
With age, our blood vessels tend to lose their elasticity, leading to cardiovascular diseases such as high blood pressure and other heart related problems.
Limitations of the Wear and Tear Theory
There are many who refute the claims of this theory, since aging is not necessarily only a result of damage to the body. There are some animals who can replace parts once they're worn off. Humans can replace nails. Hence, it is the inability to replace the worn-out parts that cause aging. Further, some problems and deterioration are a result of genetic problems. Also, there are differences found in lifespans in similar species of animals.
In conclusion, we can say that the 'wear and tear' theory does not provide a foolproof answer to the question as to why we age. Though the theory may hold true in some accounts, yet, it is not particularly true in all cases.