The 20 primary teeth ( also called deciduous or milk teeth ) usually begin to erupt when a baby is about six months old . They start to be replaced by the permanent teeth when the child is about six years old . By the age of 20 , most adults have a full set of 32 teeth although the third molars 9 commonly called wisdom teeth ) may never erupt. While teeth help people to speak clearly and give shape tooth face , their main function is the chewing of food . incisors and canines shear and tear the food into pieces premolars and molars crush and grind it further . Although tooth enamel is the hardest substance in the body , it tends to be eroded and destroyed by acid produced in the mouth during the break down of food .