There are lots of contrasting theories about the reason why we grow old. However, most of them fall into two categories Stochastic and Programmed theories.
These theories are also known as wear-and-tear theories of ageing. The basic premise of this theory is that we grow old because of the cumulative damage to our bodies from external as well as internal sources. Since, such damages are not repaired we simply wear out with time.
One Stochastic theory emphasizes the role of free radicals. Free radicals are unstable because they have lost electrons. According to this theory, and supported by modern science, these radicals are being continuously produced in the body because of various metabolism. Once came into the body these radicals react violently with other molecules in cells and in the process, cells got damaged. Further, these radicals affect the basic aspects of cell maintenance and structure and the damage done becomes irreversible when it affects the DNA. This damage increased with time thus producing changes in the body which we recognize as ageing process.
Another Stochastic theory proposes that ageing occur because DNA got damaged because of various internal and external factors. Internal factor is the unusual cell division. When cell division goes wrong the damage done is irreversible in most cases. The external factor is the damage done because of viruses or pollutants, toxins in the environment.
There are some indirect evidences of stochastic theory. We can see people showing early signs of ageing who are engaged in not-so-healthy activities like using drugs, alcohol or working in chemical industry without proper protection.
These theories put the prime reason behind ageing is in genetic programming. Theorists falling in these categories say that every human being falls in a genetic programming with built-in biological clocks. The biological clocks in turn, regulate the ageing process. However, curiously, it has been asked about the location of this clock. Recent findings suggest that this clock may be located on the strip of DNA that caps the end of our chromosome that is the teleomeres. Each time a cell divides, the teleomeres naturally become shorter, when this shortening process of teleomeres reaches a critical point, then the division of cells stop and the sign of age comes out.
Support of programmed theories can be found in the fact that each species have a fixed life span. This means the DNA of that species is programmed to wind down after certain amount of years.
At present, scientific community believes that ageing happens because of a complex interplay between environment and genetic factors. As for these two theories, not sufficient evidences are available to claim which one is exact.