When jurisdiction has been obtained by the court in the case of any child, such jurisdiction, which includes the authority to suspend, reduce, modify, or dismiss the disposition of any juvenile adjudication, may be retained by the court until such person becomes 21 years of age, except when the person is in the custody of the Department or when jurisdiction is divested under the provisions of § 16.1-244. In any event, when such person reaches the age of 21 and a prosecution has not been commenced against him, he shall be proceeded against as an adult, even if he was a juvenile when the offense was committed.