tl;dr: The older ones are better able to fight bacteria, and have a "lower ratio of neutrophils to lymphocytes in the blood", which ratio indicates stress. The

tl;dr: The older ones are better able to fight bacteria, and have a "lower ratio of neutrophils to lymphocytes in the blood", which ratio indicates stress. They evolved to live a long time because good breeding conditions are rare. No word on biochemical mechanisms.