Since I started studying biology/neuroscience I have always been puzzled by the fact that no one has yet been able to explain why we age. This question may not have a simple answer. Perhaps it is one of those questions (like "what is life?") that will never really be answered, but rather re-formulated.
My own conclusion based on what I have read until now is that many different factors are involved in aging, including but not limited to 1) buildup of toxic materials in the body (from the environment as well as waste from your own body and 2) damage to DNA.
One related mystery is why women live longer than men? What is the cause of this unfair order? Until I read an article in Scientific American I always assumed that guys simply do more stupid things that sometimes kill us... The next explanation that comes to mind is that maybe our lives are just tougher, after all we always did all the difficult things while the women merely sits in their chairs, gossiping, problem solved, right?
Apparently not. It turns out that guys stupid behavior cannot account for the difference, and the latter explanation also fails because guess what, girls have been working to. Also, in the western world today men and women do pretty much the same things, and still we die earlier. Why?
The article in Scientific American suggests that the reason we die and the reason women live longer is that our self-repair mechanisms break down and that women's repair mechanism last longer and are more effective than men's. It is as if, after a certain number of decades, the body says that this body is now so crappy that it is no longer worth fixing, you will now be a burden for others so I will not repair you any longer.
Why would women have better repair? Because they give birth, and giving birth can be traumatic for many parts of the body. In order to recover from childbirth women need very effective repair mechanisms and as a side-effect of that, they also live longer...