Aging is a gradual process by which sexually-reproducing organisms lose their youthful capacity for homeostasis. It does not normally begin until the completion of a characteristic interval of reproductive competence during which a species rears its progeny to independence. As a result of aging, older organisms are increasingly vulnerable to a wide variety of age-related diseases, ultimately culminating in their death. The trade-off between aging and repair processes is complex and observed to operate systematically within a hierarchy of at least seven different interacting levels: molecules, organelles, cells, tissues, organs, organ systems, and the entire organism.
Why we age: the disposable soma theory
From an evolutionary standpoint, the main job of any organism is to transform energy available in the environment into the maximum number of progeny. Part of the energy is used in maintaining the organism's somatic (body) tissues and part is used to propagate the germ-line tissues. Natural selection favors genetic combinations that gives the best trade-off between these two forms of energy utilization in terms of maximizing evolutionary fitness (reproductive performance). Consequently, less energy goes toward somatic maintenance than would be needed for the indefinite survival of any individual. Immortality of the germ line is achieved at the cost of death of the individuals.
Daily maintenance – through a host of systems such as DNA repair, antioxidant defenses, and protein turnover – is essential if the body is to retain its healthy functions. However, these processes are metabolically expensive. The disposable soma theory suggests that under the intense pressure of natural selection to make the best possible use of natural resources, our genes settled for maintenance and repair systems that were good enough to keep the body in sound condition for as long enough as it might have been expected to survive in a natural world full of hazards.
Although such a trade-off worked for our distant ancestors, who had a life expectancy of around 30 years, they are not so well suited to today's much safer living conditions, when we can expect to survive much longer. The upshot is that we age and die because of the intrinsic limitations in our maintenance and repair.