Aging in the human body is caused by a number of factors, but the one most identifiable factor is something called telomeric reduction. On the end of each strand of DNA in your body is a long section of “junk” DNA. This DNA does not code for anything in particular so there is no real immediate harm if it is lost. The problem is that every time a cell divides, it must also divide it’s DNA to give each daughter cell a full set of genetic material. Every time a cell divides, a small piece of the telomere is lost. Without going into too much detail, this is basically because the protein in charge of duplicating the DNA, grabs onto one end and starts duplicating the sequence, only it can’t duplicate the part that is lost because it is holding onto it.
So in any line of cells there is a genetic clock ticking based on how often the cells divide and how long the telomeres are. Eventually the little piece that is lost turns out to be something important that will cause the cell either not to function properly or it will die. Keep in mind that all the cells are doing this independently, so it is not as if all of a sudden, all the liver cells stop working, but gradually less and less of them work properly.
Causing increased division rates in a cell line will therefore cause it to age faster. For example if you like to lay in the sun and get sunburned, the increased division of cells required to repair that damage will cause your skin to age faster.
In order to eliminate aging and potentially extend human life indefinitely (which is not even a remote possibility with todays technology and medicine) one would have to first eliminate telomeric reduction, or find a way to stimulate cells to replace the DNA that is lost in cell division. In the human body there are already two types of cell that have accomplished this the first are reproductive cells and the second are cancerous cells. So only enough we may find the key to immortality in one of our most feared diseases.
The major problem with this is that in the attempt to prolong our lives we would more than likely open ourselves up to a much greater risk of cancer and kill ourselves as a side effect.
The other option would be to keep cells alive longer and slow their reproduction so that aging is effectively slowed. The problem with this approach is that cells in our bodies are programed to reproduce and die off to respond to changes in our body and outside stimuli. By slowing their reproduction we would limit the bodies ability to respond to its environment.
In short any way we try to slow or eliminate the process of telomeric reduction will either put us at increase risk of cancer and probably cause us to die early, or it will limit our bodies ability to respond to its environment and probably cause early death as well.
Right now the limit that this biological clock allows us to reach seems to be somewhere between 100 and 115 years provided that the body is cared for. Because the human ability to procreate rapidly decreases after the age of about 45 or 50, there is not evolutionary advantage to living longer as you are out of the gene pool by the time it is determined that you have a long lifespan. If we wish to propagate the genetic that lead to longevity, we would have to start a new Eugenics movement and artificially select for people whose parents and grandparents lived to an old age. Due to the social implications and the problems created by past Eugenics movements, or society will never let us engage in this kind of human breeding program, so I will predict that that 100 to 115 limit holds steady until there is some drastic cultural or environmental change which affects it.