The word metabolism comes from the Greek root "change" and involves the building up or breaking down of molecules to be used by the body. These molecules will be used for cell growth, fuel and other processes. If the process involves breaking down molecules into simpler ones, usually to release energy, it is called catabolic. If building up complex molecules, for structures or energy storage, it is called anabolic.
Developed and less active bodies do not need as many energy calories as developing and active bodies. Developing bodies are adding cells at a rapid rate, and this requires an infusion of both new material (molecules) and energy. Active bodies, likewise, need more calories to sustain the energetic demands of more motion.