In young people, the vocal physiology is not fully developed until students get past puberty.  If they are working to try and emulate an adult solo voice (which is how we learn anything that we do aurally – emulate it) chances of damage are high.  Even the best of teachers cannot see physical damage that is interior.  Also, the literature used for such lessons is usually more advanced in vocabulary and life experience.  Another reason why it is hard to start serious study before puberty is that the child’s ability to concentrate solely on the aspect of vocal production for the length of time they need to in practice and study is not yet in place in a child of that age.