Roman copy of an original, dating to maybe 470 BCE
Portraits are, generally speaking, from a later date. This piece, however, looks very much as if it could have been modeled from a real guy. And, what a guy! Themistokles rose up in Athenian society through guile and force of will; he was apparently an arrogant son of a bitch, but his forceful arguments on behalf of naval expansion and his brilliant leadership led to the destruction of Xerxes' naval fleet at the Battle of Salamis.
There were lots of portraits done at the time, and later, but there is good evidence that they were done without specific reference to what the model actually looked like. Although we don't know if this is a portrait, it looks like it could be.