Valentinian II (Flavius Valentinianus), emperor of Rome (375-392), is murdered at 21.
The son of Emperor Valentinian I, he was the half-brother of Valentinian's other son, Gratian, with whom he shared the title "Emperor". At the age of four, Flavius Valentinianus was named "Augustus" by his father's generals when Valentinian I died during a military campaign in Pannonia.
Valentinian II never exercised any real authority while emperor, being used instead as a figurehead for others. In the end he appears to have been assassinated by Arbogast, a Frankish general of Theodosius I, because Valentinian had reached an age where he was trying to make independent decisions for himself.
He even tried to dismiss Abrogast which the general simply ignored because he had been appointed guardian of Valentinian II by Theodosius and he recognized no other authority over him — not even someone with the title "emperor."