According to this theory, the rate at which we age and grow old is predetermined. The genetic material that controls virtually every function of our cells, the theory says, also controls how fast the cells age and when they die. This theory stresses on the benefits of growing old: ageing, growing and dying are essential processes because they prevent over population and allow the young to strengthen society. As one group of people ages, younger and stronger persons take on their roles in the community, bringing new strengths, better genes and novel ideas.

According to this theory, our cells— the units of life—continue to divide and replicate themselves throughout our lives, but there is a limit to the number of times they can divide. After they have replicated about fifty times, they stop dividing and wither out. This is their old age.

For cell division and replication, food and energy are required. If this is in short supply, cell division is slowed down. On the other hand, if the supply of food is more than required, the process of replication is hastened and figure of fifty is reached earlier. It means that too much of food high in calories brings old age faster.

According to this theory, animals grow old because of particular chemical processes. Chemicals produced as byproducts of biological activity in turn harm the very cells that made them. One such type of chemical, called a free radical, is particularly destructive. It attacks the basic building block of cells. As we age, more and more destruction occurs, until cells no longer function properly. When enough cells do not function, the organs — the kidneys, brain and the heart — begin to shut down, eventually leading to death. This theory also recognises that exposure to sunlight, ozone, environmental pollution, smoke, etc, play havoc with cells by destroying them.

Additionally, each species produces and resists free radicals differently, which is why some live longer than others. Genetics makes individual and families more susceptible or less, which explains why some people live longer than others.

Anti-oxidants like vitamin A, C and E found abundantly in fresh vegetables, fruits and nuts reduce the damage done by oxygen-free radicals and thus prevent degenerative diseases and retard the process of aging.

The exponents of this theory claim that a living organism is a biological machine which is highly sophisticated and complex mechanism with unique functions and substructures. It has nothing in common with a man-made machine. It is basically built up of bio-chemical, bio-electrical devices, and is self-producing, self-sustaining, self-adjusting, self-healing and is fully automatic.

For the proper maintenance of the machine, the system exercises a shrewd control over the energy part of the mechanism since it is this energy obtained by the metabolic process that keeps the machine working. It keeps a definite portion of energy in the form of reserve which it does not spend under normal circumstances. It exercises an expert control on the expendable portion of energy also. It also sees that the reserve energy remains intact and the expendable portion is always recouped. To ensure these conditions and to avoid all possible means of the dissipation of the energy, the system employs — in times of need — various protective devices like fatigue, giddiness, swoon, sleep, etc, and transports itself from the conscious state (maximum activity state) to the unconscious state (minimum activity state).

During these changeovers which occur due to illness or under condition of malnutrition or overwork, the non-vital organs which are the main energy dissipators in the system are affected. First the eye then the voice organ, next the ear, next the sense of touch, then the muscles and so on. In the condition of prostration, the entire available energy is directed only towards the maintenance of vital organs (elementary canal, heart, lungs, brain) and also in fighting out the disease. In abnormal condition of a disease when there is minimum production of energy and maximum drain on the reserve energy, there is gradual running down of the system.

Suppose in this fight, the system is at the point of defeat with germs overpowering it then as a last resort the system wages a desperate last ditch battle with the energy. It either pours out the reserve energy in one single dose or fights a regulated pitched battle under the unconscious state (coma). If even these attempts fail, the whole system collapses and the vital organs fail to function one by one and the manifestation of life stops in the body.

New researches are concentrating their efforts on the changes that take place within the cell during the aging process and trying to find out a suitable chemical which can delay the occurrence of these changes. Already a few such chemicals are under trial. This chemical, if and when found, would be able to add more fruitful years to our life span.