Aging theories.

There are numerous theories to explain the concept. I believe that you should take into account each and every one of them, and which show different points of view, all of them valid, there are some curious, and that makes us rethink the point of view from which we can see both the pathology and the evolution of the people. Broadly speaking, there are two types of theories: the biological (try to explain why changes occur) and psychosocial (try to explain why older people have different responses in aging).

You can see more in: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3589719/