Missing or absent tail bone can be a serious issue depending on the type. Actually what happens in this is that there is developmental defect in the lower spine which is of several types like partial abscence of tail bone , complete abscence, abscence associated with surrounding structures and bones. Symptoms can be absent in mild form to devastating in its severe form. Symptomps are like poor bowel and urinary control , incontinence and poor urinary control which can appear later as in this age gruop the control is not achieved. More severe symptomps are lower limbs deformities to various levels. First thing is to assess that how much structural defect is present, which can be best picked by MRI scan. Then only the prognosis will be clearly made to you. Do not lose hope first get the mri report, there are various treatment options available for the condition physiotherapy medical and surgical.