Orthodontic treatment if it is correct the alignment or to correct the bite or the crossbite, lot of things generally, we prefer children to have all their permanent teeth erupted except that of wisdom tooth. Maybe at the age of 12 to 13 it is better that an orthodontist evaluates. However it is important that the child is under dental supervision. So there are lot of complications that are caused post eruption of a permanent teeth that can be avoided. However of it is for orthodontic treatment with fixed braces then 12 to 13 years will be an ideal evaluation by your orthodontist and after that depending upon these x-ray or growth left in the child or depending upon a lot of parameters a dentist will evaluate as to when an orthodontic treatment can be started.