You might get better answers on the Biology forum down a ways, but let me give you the common wisdom.
Your chromosomes have a complex of molecules at the end called a telomere (that's Greek for end part, duh). Of course every cell in your body has those chromosomes, and those cells don't live for ever, they divide, producing "daughter cells, which inherit the chromosomes. Now here's the kicker: every time the cell divides, the telomeres on its chromosomes lose some molecules. After I think it's 22 divisions, the telomeres are all gone. And when they are gone, the cell can't divide any more. It just gets older and eventually breaks down. Finally enough key cells break down that your vital processes can't continue and it's ta-ta, baby.
Why do the cells do that? Well there are cells that don't. because something, a virus or something, has damaged them. We have a word for those cells: Cancer. Unstoppable division is even more quickly fatal to our bodies than gradual cellular senescence is.
This puts an upper limit on how long we can live naturally. Of course something else - fatty tissues in the arteries, infection, having a safe fall on us, can kill us earlier. And there are things we can do to keep our cells healthy longer. I take a lot of vitamine E and C, which are thought to clean up some of the waste products that happen to cells as they age. No guarantees, but it has helped others.