I don’t think we really know. It seems like the maximum a human can live is about 120 years with an average of about 80. For a tree however, the average might be 150 years, but they can live to be over 4000! There seems to be some form of biological switch which tells our body that it is too old, but we don’t know what it is.
All cells have a certain life span and when they reach the end of that span they die. While we are alive we replace those cells with new ones (Red blood cells live for about four months, while white blood cells live on average more than a year. Skin cells live about two or three weeks. ) Quite why we’re not sure. Sometimes the reason we die is that one kind of cell will stop being replaced and the organ can’t fix itself
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