Generally speaking, for patients with simple tooth deformity and mild facial skeletal deformity, correction generally begins after the replacement of deciduous teeth. Boys are generally 12- 14 years old, and girls are 1 1- 13 years old. At present, children's primary teeth are generally changed later, so the best time is after all primary teeth are changed. However, some serious patients (such as maxillary protrusion and mandibular retraction) should be treated as soon as possible. Considering the cooperation of children in treatment, it is recommended to start treatment at the age of four. For these patients, if they don't start to correct their teeth until the tooth replacement is completed, they may develop into serious facial skeletal deformity, which greatly increases the difficulty of correction, and some even need surgery to solve it.