Sabo was 10 years old when Dragon took him under his wing. When he joined the Revolutionary Army, he had already accepted their ideals. So Hack, who was the training instructor for orphans, decided to teach Sabo as well. But unfortunately Sabo was already so strong, that there wasn’t anything Hack could teach him.. Finally, Sabo ended up taking the training from Dragon himself, and kept on becoming stronger and stronger along the years.
When Sabo was 13 years old, Koala (age 14) joined the Revolutionary Army. Hack and Koala took good care of Sabo and they often undertook missions as a team.
Source: SBS Volume 80