According to scientists the answer to this question lies within the manipulation of DNA.
Extending our life expectancies is a hot topic at the moment and recent studies seem to suggest that a break-through is ever closer. A group of scientists in California managed to create a mutant version of Yeast which can live up to ten weeks which is 10 times longer than the usual 1 week expectancy. Although the difference between humans and Yeast is obviously huge, it is undeniably a step forward and the scientists involved have suggested the work is relevant to humans.
It is a generally accepted by the scientists who study ageing that the maximum lifespan of a human is no more than 125 years, although in contrast to this the scientists who created the mutant Yeast where quoted a saying, "I don't believe there is an upper limit to the life of any organism". A theory by many experts as to how we can live forever is by the replacement of major organs, this would involve the growing and replacement of organs to counter the effects of ageing.
Many experts don't believe the process of ageing can be avoided and for people to live for up to 10 times their natural life expectancy is impossible. Yet the scientists of California's research suggests that organisms can be pushed beyond their natural limits so maybe this can be applied to humans. To achieve this extraordinary feat the scientists deleted two genes which are known to promote ageing in Yeast and put the micro-organisms on a low calorie diet.
This idea has got many scientists thinking and a theory has been proposed as to how much DNA can influence aging. One of the theories was that DNA can keep mending the body and organs but it becomes increasingly costly in terms of resources and energy used. So when it reaches a point where it is no longer worth-while for the body to continue this restoration it simply stops, but from the view of the person's Genes it is irrelevant as the person will have most likely reproduced.
A low calorie diet is also a well known way to increase the life expectancy of many organisms, although it is debatable as to whether this is the same in the case of humans. A theory as to why calorie restriction results in a longer life is that it puts the organism into a state where most bodily processes are shut down apart from the basic necessities, so less energy is wasted.
Let us imagine if we could live forever, would it really be that great? The world would become over-populated very fast unless controls were put in place which would still only delay the inevitable. Another theory which has more realism in terms of common sense than scientific backing is that we age and die simply to make way for future generations. So maybe we can invent a way for us to live for another 100 years or so, but forever? I doubt it and that's fine by me.