Aging is best explained by declining mitochondrial function over time.

  Aging is best explained by declining mitochondrial function over time.   This answers its own "why."   Mitochondria produce the energy needed for repair so if any of it gets lost it sets up a vicious cycle. And some *always* gets lost.   But how much is under your control.   From Joe Rogan Experience JRE 2420. https://www.youtube.com/watch?v=QBn54YNnKD0   It's a vicious cycle initiated by the second law of thermodynamics which requires a constant input of energy to prevent the collapse of order.   Hence, there will always be slippage, but how much slippage depends on genetics, environment, and behavior, and all slippage leads to loss of mitochondrial repair, which itself directly causes the loss of repair capacity as it weakens the defense against the imperative of the second law of thermodynamics.