The best time for children to have their teeth straightened is around 12 years old, but there are two special cases that require early orthodontic treatment, the first is the period of milk teeth at the age of 4 to 5 years old, and the second is the period of tooth replacement at the age of 6 to 12 years old. 1. As children aged 4 to 5 years old are prone to jaw reversal, early orthodontic treatment is needed to prevent underdevelopment of the maxillary bone and to prevent the permanent teeth from further reversing their jaws. If children develop bad habits such as sticking out their tongues and biting their lips, they should also be corrected in time. 2. At the stage of tooth replacement, i.e. 6 to 12 years old, girls are mostly at the age of 8 to 10 years old and boys are mostly at the age of 9 to 12 years old. Some children like to stick out their tongue, bite the lower lip and other anti-mandibular state, it is best to go to the hospital for examination and evaluation. Determine whether it is simply a bad habit or the teeth have formed a bony anti-mandibular, clear diagnosis and then treated under the guidance of the doctor.