How long can people live?

One thing that is often overlooked when considering human longevity is brain cell death. The brain loses roughly 10K neural cells per day. These neural cells do not regrow and there is at present no way to restore them.

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Quote: "show a consistent decrease in the volume of the brain after about age 40. It is likely that this “shrinkage” derives much more from the loss of white matter,"

So, by the age of 80, the typical person has a brain mass similar to that of a young child. Even if you lived forever, your brain would eventually peter out.

There is some hope for regeneration through the hippocampus and the subventricular zone: http://en.wikipedia.org/wiki/Neurogenesis

The hippocampus is widely known to have regenerative cells but the subventricular zone was not understood until recently. Still, these have almost no impact on storing memory, so in the sense that they would prevent you from losing cognitive function, they are worthless. To use a networking analogy, the hippocampus is like a router and it is periodically replaced when it goes bad (cells die) but the network on which it resides is losing computers everyday. Eventually, the router will be around, but all of the computers will die.

"Neurogenesis is substantially reduced in the hippocampus of aged animals, raising the possibility that it may be linked to age-related declines in hippocampal function. Given that neurogenesis occurs throughout life, it might be expected that the hippocampus would steadily increase in size during adulthood, and that therefore the number of granule cells would be increased in aged animals. However, this is not the case, indicating that proliferation is balanced by cell death. Thus, it is not the addition of new neurons into the hippocampus that seems to be linked to hippocampal functions, but rather the rate of turnover of granule cells."

This is basically saying that cells regrown in the hippocampus have no function other than hippocampal function and thus aren't useful for repairing the damage caused by aging.

Regarding the SVZ, "... the SVZ can recover itself following mild injury, and potentially provide for replacement cell therapy to other affected regions of the brain." This is a great area of research, but so far as we can tell, this has very weak restorative abilities. We do have hope that these stem cells can be used in therapy.

There is hope for using stem cells and restorative techniques. But at present, brain cell death is a byproduct of living. In speaking with researchers on this matter, many estimate ~130 years for over 50% of your brain to be dead. This is a lot longer than most people live, but their cognitive abilities will have declined to a point to make the exercise dubious.

Eventually, your brain would die entirely, but I suspect in a non-linear fashion. You do need a certain 'critical mass' to do things like language and math puzzles and once you decline to ~30% remaining mass, you would probably lose all ability to do even basic problem solving.

I think that we all wish that there were a way to regenerate cells in the brain more readily.