Normal people have two sets of teeth in their lifetime, namely, milk teeth and permanent teeth, with 20 milk teeth and 28~32 permanent teeth.
1. Milk teeth erupt at about six months of age, and all of them erupt at about two and a half years of age. According to their morphology and function, there are 20 milk incisors (8 in total), milk cuspids (4 in total) and milk molars (8 in total) in total.
2. Permanent teeth begin to erupt at the age of six, and all the milk teeth are replaced by the age of twelve; they include incisors (8 in total), cuspids (8 in total), premolars (8 in total), first molars (4 in total), second molars (4 in total), and third molars or wisdom teeth (0~4), which amount to 28~32 teeth in total.
3. The third molar of the permanent teeth is more variable and will not erupt until adulthood. Some people cannot erupt all of their wisdom teeth or even not erupt at all, so there is variation in the number of teeth.