Aging is a natural process that we all go through. It is an inevitable condition that animals and humans alike share in common. The mechanism of aging is so complex such that neither science nor any forces of nature can dictate its natural course. There might have been several attempts to alter or delay its course, but they do not offer lasting and long term effects. In the end, we still age, which ultimately culminates in death.

Up to this time and age, aging is still not fully understood. What scientists know is that aging is affected by an interplay of several factors, some of which still need to be studied. So far, we have the following theories that attempt to explain why we age.

1) Gene Theory of Aging.
             This theory is derived from Darwin’s evolutional theory of natural selection. It 
              supposes that we inherit genes that limit our lifespan amongst other species.

2) Theory of Programmed Cell Death
 All cells in our body are made up of chromosomes which contain the genetic material, called DNA. This theory suggests that each time a cell divides, these chromosomes become shorter, thereby limiting the number of cell divisions. During infancy and childhood, cell divides rapidly. But as we age, cell division slows down and eventually stops.

3) Aging as a Consequence of Oxidative Stress
 When our body is subjected to stress (e.g. illnesses, toxins, chemicals), we produce a lot of free radicals in the body. These free radicals can damage the cells and organs of the body, causing premature cell death.

4) Aging as a Consequence of Hormone Losses
 Hormones are chemicals in the body which are vital for bodily functions. Take for instance, estrogen which is a female sex hormone. Aside from its reproductive function, estrogen has also a role in maintaining bone density. Hence during menopause, when estrogen is low in women, the risk for osteoporosis which is common in elderly women is high.

5) Aging as a Result of High Energy Consumption 
 The energy consumption hypothesis says that burning more calories can increase damage to cells, resulting in more rapid aging. On the other hand, severely restricting calorie intake can prolong life and slow down the progression of age related symptoms. Thus, the slower your body metabolism is, the longer your life will be.
 
Again, these are just proposed theories that need thorough scientific research and it will take many more years before we can fully understand the complexities of aging.