Theories of Primary Aging
What is ageing and why do we age is both a philosophical and scientific question, and both have more than one theory. Perhaps we will explore the philosophical theories in another posting, but, for this posting we will discuss the scientific theories.
The mitochondrial theory
      Mitochondria is the "powerhouse" of the cell. It is described as that because it produces the energy required for any and all activity in the entire body. No mitochondria...no movement, no life. The video below explains in more detail how it functions.
 Telomeres theory
         This next video is a long one, but check it out. The speaker has a point of view that differs from what I have learned in the past. I believe that you will appreciate it if you are familiar with telomeres and telomerase. But, if you want to view shorter videos, just fast forward to the end and there will be other videos to watch. I like this theory because I think that all the other theories of ageing corallate with it, that is, this theory can be causal to the others.
Generic theory 
          The next very short video discusses the genetic theory of ageing. We use to believe (some still do) that if you have a genetic history of longevity you are more likely live a long life, and conversely if your history has a pattern of short life, then you can expect the same. However, we now have evidence that genes have very little to do with longevity. Other factors like life style, nutrition, stress, quality sleep, etc. have the greater influence.
Parsons, P. A. (2003). From the stress theory of aging to energetic and evolutionary expectations 
        for longevity. Biogerontology,4(2), 63-73. doi:http://dx.doi.org/10.1023/
        A:1023308122587