I asked this question and I got a lot of "how" answers but no "why" answers. One Professor Beatz gave a long explanation about how the DNA is at the ends, the one strand, being continuously copied, goes al the way, while the other stand misses the last few nucleotides...The result is that you DNA is buffered with a lot of junk at the ends in order to prevent the replication process form destroying useful genes. But eventually, after many years, this gradual attenuation of the ends of the DNA will result in the loss of important information. As this happens, the aging process results in cells that don't function as fully as they should. Gradually, the loss of information from your DNA will result in the failure of your body to do what it needs to do.
OK OK OK but WHY DOES that happen? Can you answer that Professor? Turtles live over 200 years why do we grow old and die in only 80 with so much of our brain capacity wasted.
The answer is that there is a continual battle between two evolutionary strategies that exist at opposite ends of a spectrum. The strategies are called r-selection and K-selection (named for variables in the Verhulst equation of population dynamics). Each strategy is designed to maximize population under different conditions.
The r-selective strategy focuses on trying to create as many offspring as possible in as little time as possible. There is little attention given to the strength of the individual offspring, and the perpetuation of progeny is accomplished by sheer numbers. A good example of this is seen in cockroaches and spiders. A mother might lay thousands upon thousands of eggs, but most of the eggs will not survive until maturity. In fact, in some species the mother might even devour some of her own young, destroying the weakest who cannot escape while the strongest press on. Typical of the r-selective strategy is high numbers of offspring per parent, short lives, rapid approach to sexual maturity, and high mortality rates (especially among those not yet sexually mature).
The K-selective strategy is the opposite. Each parent will have a few offspring, but they will invest a lot in each offspring (metabolically, behaviorally, etc). The Emperor Penguin is a good example of the K-strategy (relative to most avian species). The K-selective species have delayed sexual maturity, lower mortality rates, and fewer births per parent. These organisms will live a longer time.
There are forces that pull a species in one direction or another. If the species is operating at or near the carrying capacity for the species in the habitat, then the drift will be towards K-selection. If high mortality is inevitable, then things drift towards r-selection.
In the human world, we see that these sorts of strategies are moving in different directions in real time. In China, the one-child policy is pushing things towards K-selection by creating higher investments in the children (ie parents in China take much more active roles in their children's lives in China today that was possible when concerns were divided among several children), while in Western Europe the legislative actions that have forced greater investment in children (ie education and labor laws) have pushed the K-selective strategy of reducing the number of children (the fertility of Western Europe is extremely low). Conversely, in war-worth places, people are more sexually active at younger ages and have more children, even as infant mortality soars. In America today, one can see the different approaches between the upper class families which likely have children who will each have college funds and will marry late and have few siblings, compared to larger families in the lower economic ranks where families are likely to be big, kids will have more need to take care of themselves (or even their parents) financially, and sexual activity and pregnancy will likely come at an earlier age. This is typical of r-selection, and these people will likely live shorter lives.
The answer then is that the length of the human life is balanced by these things. Epidemics, crime, and war will shorten the life by means other than genetic means. But as this happens, the species will only grow if it is able to reproduce quickly and frequently. While this is happening, natural selection is focused on making the species reproduce quickly, and no "effort" (for want of a better word - please forgive me for anthropomorphizing the idea of evolution, but it helps to think of the trends in terms of directed action, even though that is not how evolution works) will be directed toward many quick births.
Once you pass the age of sexual activity and/or fertility, evolution cannot work on you at all. Evolution functions by making more offspring in those that successfully reproduce than those that don't.