Comment Not an upper limit (Score 5, Informative) 333
If you live long enough most of your cells end up dieing or critically damaged by the formation of inclusion bodies caused from misfolded proteins. As far as we can tell the cells are otherwise fine they are just slowly accumulating that damage over time. This is also what alzheimer's is. The problem is that misfolded proteins are kind of contagious to other proteins in the cell and that is what leads to the inclusion bodies.
We are making progress though on being able to clean out the inclusion bodies. Your cells do have the ability to take them apart but somehow they end up not doing it. Give us some time though and we will fix this problem also and clean out these inclusion bodies in all of your cells and then your cells will work much better.
The other issue we need to fix is activating telomerase to extend our telomeres. The basic issue is that natural selection does not really select for anything after reproductive age so humans are filled with a bunch of small defects and we are getting better at repairing the damage. I really look forward to what can be done with CRISPR-CAS9 to repair DNA damage and replaced damaged genes.