Children can only grow 20 temporary teeth; the upper and lower jaws both have the same 10 teeth. Around six months of age is when a baby's first set of teeth begin to break through the gums. Lower incisors are often the first primary's to emerge. By the age of three, most children will have all of their temporary teeth. Between the ages of 6 and 12, children often lose their first set. They are eventually replaced by a permanent set. The first to erupt are often the molars. By the age of 21,