Put them to choose his team on his own and BAM you have a leader. XD
But honestly i think oldest and longest trainee isn't always the best leader, neither the most talented. People might not recognize it, but being a leader needs a type of personality too. I have been in the fore front of some groups in organizing and stuff, for school and for summer camps and while i did do well technically i ABSOLUTELY hated it, and was stressed beyond belief when things got fucked up. A good leader is a good problem solver, a person who can unite opinions in a group, a person who brings the best solutions to a problem by listening to every individual and combining their perspective on it.
For ex. Stray Kids members always say that Chan is a good leader because he is focused on his work, but the team is always a priority. If the members feel uncomfortable about smth he is there to bridge that communication distance between the team and the company. I think that's admirable.