Every demon will come to be aware of its weakness in the demon hierarchy. How they're dominating the human race and other creatures was just child's play compared to what the true demons at the top had.
This came around the age of 1 year old when usually a demon has enough spiritual energy to further their spiritual body cultivation and take control of their path. That was what happened to the Leopard demon after it had consumed the spiritual energy of ten thousand humans from a town.
Typically demons are not able to grow their spiritual energy after they are born because spiritual energy doesn't grow after a creature is born much less when it's not exposed to the harsh environment and isn't stimulated. This protection from the outside elements allows demon spiritual energy to never decay or have to face the terrifying death spiritual energy that occurs when a creature is close to the end of its lifespan, allowing them to live forever.
