It is the  withdrawal (extraction) process of  third molar tooth  having smear problem usually within the mouth. As these teeth are has time to last between the ages of 17-25 it is also called tooth of age 20. This occurs  where the teeth are left in the mouth. If it lasts  properly in contact with opposing teeth in one direction, and does not cause place  obstruction ,age of 20 doees not need to be  pulled.  Dog (canine) teeth is also kind of tooth  having  a problem of lasting.Those teeth can also be pulled out of the palate or lasted with  orthodontic brackets.