Why do we die of old age? i'm not looking for answers like 'the organs shut down,' or 'our body stops producing genes.' i'm looking for why that happens. we have the ability to stay alive forever by having our cells replaceing themselves all the time. so why does it stop?

Nothing has the power to live infinite. it is a natural process because everything expires at some time. we all have to die or else the whole system of the world can not work. it is a whole ecological cycle that has to be followed. if we do not die and live forever then the new generation will come and come and come and no one will die. the whole world will lack in space. when the previous people leave then only the new could come in the space. A whole lot of science applies to do this beyond organ and cells. we all die because our life is like a bubble it can be exploded at any time. this world is just a place for us to perform a test which is being taken by God so that we could succeed on the Day of judgment and go to heaven. I hope you have a clear answer now.  

Icant be totally sure but i would say with confidence that it is example #1 about cells.