According to WebMD, Shih Tzus are one of the longest living dog breeds, with an average life span of 11 to 14 years. One of the reasons Shih Tzus have such a long life span is that the breed has few congenital issues aside from eye disease, hip dysplasia and patellar luxation, a dislocation of the kneecap. None of these issues are life-threatening. The only serious congenital disease that affects Shih Tzus is juvenile renal dysplasia. This is a rare condition that causes the kidneys to develop abnormally and can lead to serious health issues later in life. To avoid buying a Shih Tzu suffering from juvenile renal dysplasia, seek a reputable breeder who is able to provide a health certificate.