The time of tooth replacement in young children is usually 5-6 years old, girls change teeth earlier than boys, and children with high height, heavy weight and good nutrition are earlier than children with low height, light weight and poor nutrition. At the age of 5-6 years, the mandibular milk incisors begin to loosen and the crowns of permanent incisors are exposed in the oral mucosa and gradually move to their normal position. Due to the significant difference in the size of the crowns of the milk teeth and permanent teeth, temporary crowding may develop. At the age of 10-12 years, all 20 baby teeth are lost and replaced by permanent teeth, while the second permanent molars are grown and permanent teeth are established.