According to the American Academy of Pediatrics, or AAP, having a six pack at a young age does not stunt growth. The AAP says that a child is able to build strength effectively in order to prepare for and excel in sports while staying healthy.
Most injuries in children's sports are caused by a lack of strength training, and most kids do not do strength training because of misconceptions and a lack of resources. The AAP has released an app called IronKids. It contains a workout with strength training created by Dr. Jordan Metzl, a sports pediatrician. Dr. Metzl states that it is safe to begin strength training in children at age 8. However, check with a pediatrician before a child begins any fitness regimen.