Mantak Chia suggests that it is (lack of proper flow and not enough of) energy which is responsible for aging, e.g. in the "Awaken healing energy through the tao":

As we age, the energy routes which bring vital power to our internal organs and enable them to function become progressively more blocked by physical and mental tension. The result is general fatigue, weakness, and poor health. A young person usually has sufficient level of energy to keep the routes less obstracted so that the power still flows. The organs are thus nourished, and there is little sickness. But if we do not live healthily and practice to keep the routes open, they will gradually close and cause emotional imbalances, premature sickness and old age.
From my point of view, energy is not the cause, but the symptom of aging, just like the whole body and its cells are gradually get more and more degenerated. This means that there must be a real cause for the life force, and energy, flowing is just the effect - IMHO it is related to higher levels, not just to the etheric level which is used by Mantak for the etheric energy (called chi in his books) in his practices to be circulated by meridians.