the cells which die are replenished by our body but with age the capacity for renewal goes down while some cells like those belonging to brain can't be renewed at all, so in the end cells die consequently tissues die then organs become weak and ultimately the person expires.
That is because you have a certain cell in your body, called an Ambirone, they help you to keep you healthy and to keep your imume system high. But at a certain time, those cells stop re-producing and you will go weak
usually bloodcells are aging, so they carry less nutriant to the other cells of the body, wich drives to organ disfunctions. if we can solv the problme of the blood-aging, maybe we could live for 200 years or more, but basically all cells copy in bad quality. there are lifeforms on earth wich never die, like tipes of jellifish.
Mankind was originally created to live forever, however, according to Genesis 2:17 - "But as for the tree of the knowledge of good and bad you must not eat from it, for in the day you eat from it you will positively die." Accordingly, Adam and Eve sinned and we inherited old age and death.
Romans 5:12 - "That is why, just as through one man sin entered into the world and death through sin, and thus death spread to all men because they had all sinned." In saying that death “entered into the world,” the Bible suggests that mankind was not originally meant to experience death. For humans, old age and death are the result of the first man’s rebellion against God
This defect—sin—was also passed on to Adam and Eve’s offspring, the entire human race. In a sense, it was like a hereditary disease. Not only did Adam lose the opportunity to live a life free from the scourge of death but he also transmitted imperfection to his offspring. The human family was taken hostage to sin.