Be patient as they develop and give them guidance yourself. I think you'll be disappointed. Most kids at this age are good followers and just learning how to lead. Their personalities are just beginning to blossom. They are only beginning the process toward adulthood of discovering who they are.
Leadership is a trait which requires both physical and mental maturity.
You will have some players will be physical leaders and others who might be intelligent leaders but your best results in terms of a team having someone step forward is someone who can do both. I believe as well that your leaders must be of strong and honest character and you can encourage someone with these tendencies by giving them a little more responsibility in helping you take care of your team responsibilities. You may not have one player that comes to mind, but if any one of your players has these traits that is who I would work to develop into a leadership role. They may not be your best player, but they should be one of your better players. Part of leadership is the ability to lead by example.