The same tooth lost twice will not grow back. 1. During a person’s lifetime, there are two sets of natural teeth, which are categorized as permanent teeth and milk teeth according to the time of their eruption. 2. Milk teeth usually start erupting gradually from the sixth to the eighth month of a baby’s life, and all of them will erupt by the age of two and a half. 3. At around the age of six, sometimes earlier than five or later than seven, the first permanent molars begin to erupt, along with the permanent incisors, and the rest of the permanent teeth will gradually erupt, and the milk teeth will then fall out naturally. Permanent teeth will not grow back once they fall out. Therefore, you should pay attention to protecting your teeth in your daily life, and go to a medical institution in time if you have any discomfort in your teeth or mouth.