Answer:
Physical appearance is important to adolescents, so it's natural for teens and pre-teens to want to start working out. Skinny beanpoles or pudgy babies who grew into pudgy kids may start having visions of building muscles to re-shape their bodies. Before puberty, however, kids' bodies aren't ready or able to build significant muscle mass, so if they attempt to gain muscle before their bodies are ready, they are likely to be disappointed.