We can easily see the similarity of the above picture. If we examine carefully we could detect the differences; a child's legs moving very fast whereas an old man legs hardly move an inch. A child's body is soft and flexible, an old person body is stiff and rigid especially the joints. When a child is born and it was young, yang qi keeps growing say at 15 years until say 75 years then yang qi will stop growing. So from a not fully grow child state to a fully grown old man state, in my opinion the effective useful healthy life span of human is 75 years although statistics shows human can live until 120 years old with good health, they are minority. Getting old is a phenomenon where yang qi (energy flow) in a body keeps diminishing. When yang qi deficiency begins with the legs, the first sign of weakness is a person can't walk properly. We could easily recall our experience as a child learning to walk, an old person learning to fall. ...
