Most infants start teething at 6 to 8 months of age, but there are individual variations and teething within a week is generally considered normal. Milk teeth generally begin to erupt before the age of 1 year, but there are individual differences between infants, some earlier, some later. At the age of two to two and a half years, 20 milk teeth erupt completely, at the age of six years, permanent teeth begin to replace milk teeth, and at the same time, the six-age teeth, i.e., the first molar erupts, at the age of 12 years old, all the milk teeth are replaced, the second molar generally erupts at the age of 12-13 years old, and the third molar needs to be erupted at the age of 16 years old, so that the normal adult permanent teeth are 28-32. During the period of teething and tooth replacement, it is necessary to consult a doctor in time to avoid delaying the condition.