Parents may have heard that the best time to start orthodontic treatment is during the early permanent dentition (around 12 years of age). However, if the patient has already developed a jaw disorder before the permanent teeth stage, or a condition that may affect the subsequent craniofacial development, it is necessary to remove the cause of the condition as soon as possible and start early preventive or interceptive orthodontic treatment. Reverse-jaw, commonly known as “geomandibular”, at the age of 3.5~5 years is the best time to treat the reverse-jaw of the baby teeth. Early intervention of the reverse-jaw of the anterior teeth can promote the normal development of the maxilla and inhibit the over-development of the mandible, which can avoid the occurrence of more serious bony malocclusion and reduce the trauma of dental occlusion. For patients with mandibular retrusion, functional orthopedic treatment can be utilized to promote mandibular development by taking advantage of the patient’s growth and development potential, which can also reduce the difficulty of orthodontic treatment in the future. In addition, individual teeth antimaxillary, abnormal replacement of permanent teeth, and bad oral habits also need to be corrected as early as possible to create favorable conditions for jaw growth and development. It is recommended that children start regular oral health and orthodontic checkups from the age of 2.5 years old. Timely and early intervention can reduce the occurrence and severity of malocclusion, and lower the difficulty and risk of orthodontic treatment in the future permanent teeth period. Theoretically, there is no age limit for orthodontic treatment, i.e. orthodontic treatment can be done at any age. Adults’ jaws have stopped developing, and orthodontic treatment is more difficult, risky and time-consuming than that of children, but orthodontic treatment is still possible as long as periodontal and temporomandibular joints and other underlying conditions permit.