Fabii

Fabii caesi ad unum omnes praesidiumque expugnatum. trecentos sex perisse satis convenit, unum prope puberem aetate relictum, stirpem genti Fabiae dubiisque rebus populi Romani saepe domi bellique vel maximum futurum auxilium.
(Livy 2.50.11)

The Fabii were all slain to a man, and their fort was stormed. Three hundred and six men perished, as is generally agreed; one, who was little more than a boy in years,* survived to maintain the Fabian stock, and so to afford the very greatest help to the Roman people in its dark hours, on many occasions, at home and in the field. (tr. B.O. Foster)

* Quintus (or Quinctius) Fabius Vibulanus was said to be the only male to escape the slaughter of the gens Fabia at the Battle of the Cremera (477 BC). He became consul for the first time in 467, so he can’t have been that young!