Details of Crus' younger years are not known. In 72 BC, Caesar's man Balbus acquired his Roman citizenship for service under Pompeius against Quintus Sertorius in Spain. On the basis of the Roman names he took – Lucius Cornelius Balbus – and on the basis of later letters to Cicero, it is possible that both Balbus "major" and "minor" obtained citizenship with the sponsorship of L. Cornelius Lentulus Crus, who may then have been serving with Pompeius as a legate (Pompeius was there 76 BC to 71 BC; had Crus been born c. 98 BC, he would have been between the ages of 22 and 27 at the time).