The failure to replace milk teeth in the thirties may be due to congenital absence of permanent teeth, permanent teeth obstruction, ectopic eruption of permanent teeth and other reasons, as follows. 1. Congenital absence of permanent teeth: if the permanent tooth germ is congenitally absent, there will be no eruption of permanent teeth, and the milk teeth are usually retained in the dentition to exercise their functions. 2. Obstructed permanent teeth: If the permanent teeth are completely obstructed, the root resorption of the milk teeth may not occur, and they will not fall out automatically. 3. Ectopic eruption of permanent teeth: If the direction of eruption of permanent teeth is abnormal, permanent teeth and milk teeth may exist in the mouth at the same time, forming “double rows of teeth”. There may be other reasons why milk teeth are not replaced in your thirties, so we recommend that you go to the hospital and get a clear picture of the cause.