I'm not sure if this is one of the consensus primary causes but the most compelling explanation I've heard is capillary damage.

Red blood cells and capillaries have roughly the same diameter leading to constant friction. Your body repairs the capillaries but, given enough time, it makes mistakes. As these mistakes pile up your circulatory system, your body's supply chain, slowly gets less efficient. This leads to every cell in your body gets the energy it needs a little later, and has to hold onto waste for a little longer.