Teratomas occur as pure tumors in infants and young children, in whom they are universally benign, and in adults, in whom they carry an approximately 25% risk of metastasis.Testicular teratoma in children older than 4 years is unusual. In contrast, teratoma usually occurs in adults as a component of a mixed germ cell tumor, and is present in more than half of all mixed germ cell tumors and in approximately 25% of all non-seminomatous germ cell tumours. Immature teratoma frequently occurs in patients between birth and 7 years of age (median 13 months). The immaturity of teratomatous components has not been shown to be an indicator of poor biologic behavior in the primary tumor. Aggressive behavior in testicular teratomas is more directly related to the age of the patient than to the histologic type.

 The metastatic potential of pure testicular teratoma has been a source of confusion. Children with pure teratoma are not reported to have metastases. Conversely, postpubertal patients have a definite risk of metastases, even with pure mature teratoma.