The Julio-Claudian dynasty, the first five Roman emperors from Augustus to Nero, produced many portraits of their relatives and princes. This is thought to be Drusus the younger, the son of the second Roman emperor Tiberius, born in 14 BCE. After service in the Roman army he was in line to succeed his father as emperor; but ancient sources tell us he was poisoned by his wife Livilla and her lover Sejanus in 23 CE.
Other portraits in Madrid and Naples known to be of Drusus have a resemblance to this austere and bony face, hence the identification
Rome, Vatican, Museo Chiaramonti 628
Bernoulli: Griechische Ikonographie (1901), II, 28, no.11
Amelung: Catalogue of the Vatican Museum I (1903), 733, pl. 78 (?)