Aging has a striking impact on the heart and the vascular system, and particularly on the large elastic arteries (i.e., aorta), resulting in a multitude of changes at different structural and functional levels. As result, medial degeneration (MD) occurs. A characteristic example of MD is sporadic thoracic aortic aneurysm (S-TAA), whose patho-physiological mechanisms remain unclear. In this study, typical MD morphological phenotypes were researched in S-TAA cases and control aorta specimens using histopathological and mainly immunohistochemical analyses. Three phenotypes (I, II, and III) were detected, but the phenotype III was observed. Elevated cystic MD, plurifocal medial apoptosis, and increased metalloproteinase-9 amount characterize it. In addition, it was significantly correlated with the severity of elastic fragmentation, hypertension, and smoking, and particularly with advancing age. Thus, phenotype III might represent the typical MD phenotype associated with S-TAA in old people that have a major risk of aorta rupture and dissection independently on aneurysm diameter. This might permit the assumption that phenotype III with its typical histological abnormalities is an optimal biomarker of rupture and/or dissection in aged individuals and is useful both for applying different surgical approaches and providing appropriate surgical indications.