If our bodies are always regenerating new cells why do we grow old and die?
One idea is the loss of information.
When cells are duplicated ( regenerated) the information in the cell must be copied transcribed and replicated faithfully. Any mistakes in the copying of the information results in a weakened or non functioning cell. When the cells loss information the cells "grow old" causing the organism to grow old.
It is estimated that on the average all the cells in the body die and are replaced by copies every 7 years. At age seventy the human body has undergone 10 cycles of copying and replacing with 10 opportunities to make mistakes and lose information. Hair cells lose the information to make proteins like melanin that causes hair color. So by age 70 most humans now have grey hair. This is a visual manifestation of the aging of all the cells in the body.
When the loss of information reaches the critical point where some vital body system no longer functions the organism dies. If a way could be found to use stem cells to replicate new cells with no loss of the original information to replace the cells damaged by the loss of information the organism would never die of old age. Or if a means could be found to stop the law of entropy ( increasing disorder) this would stop aging.