A common narrative in the strength & conditioning realm of the world is that children should not participate in resistance training (specifically in regards to training with free weights). However, that narrative is changing as more and more studies are beginning to prove that it is not only safe, but in fact beneficial, for children to participate in resistance training from as young as 7 or 8 years of age.
It has long been believed that children participating in resistance training would be unsafe for them largely due to the fact that their bones are not yet fully developed and lack the density to handle the loads involved. This however has been debunked, and it has in fact been proven that resistance training can actually aid in the growth and development of healthy bones in children.
The Mayo Clinic, largely regarded as one of the most recognized and acclaimed sources of medical information, has stated that "strength training can become a valuable part of an overall fitness plan (for children as young as 7 or 8) — as long as the child is mature enough to follow directions and able to practice proper technique and form."
With that in mind, when looking into resistance training for children it should be the child's physical and mental maturity, not their age, that determines whether or not they are ready to participate in resistance training.