A few thoughts and ideas...
(Pay no attention to the use of he over she, my post is intended to be gender neutral! )
Natural Leaders are hard to come by, so the young student should be encouraged to develop leadership skills. Taking charge and taking over are different things and that is where the other students will have a reaction. One will be accepted because he can get others to work with him and the other will be percieved as a bully and a know it all. Very fine line. The goal is to turn the latter into the former. Very often the difference is based on self image. The better the younger student feels about how he fits in and can contribute, the better he will relate to the students he is working with.
Knowledge and Experience vs Leadership Skills: The team leader should be the one who is able to identify those who have the knowlege and let them shine. So they need to have enough experience to know how to ask the right questions. The team leader does need to have the respect of the peer group, but that can be earned after the leadership role is assumed. (Otherwise, only the popular students will be selected as leaders even if they are not good at leadership at all.) And it is very important that a team leader want to develop leadership skills, and be willing to learn from everyone, students and mentors alike.
Our team does a few things in selecting and developing our leadership team. Team leaders are selected in the spring with several open spots remaining until the fall. There is always an opening left for a Freshman to fill in the fall along with another opening for another student who may not have felt ready to assume a leadership position in the spring (summers change students). It might not be possible to change the structure of your leadership group right now, but it is something to consider. One of our current team leaders is a FLL "graduate" and he has been part of the leadership team since his freshman year, demonstrating the knowlege and confidence that FLL had given him. Fortunately, I don't think he has a "know it all" bone in his body, so your situation never occurred.
The leadership team also does leadership training and all students are invited to participate in some of the leadership workshops. Learning about leadership helps the students discover their skills and learn how to get the best out of the others as well. We would be happy to share some of the resources that the team has used if you are interested.
Without changing your team structure, I'd suggest that the Team Leader take the younger student under his / her wing. Help the student take over planning / leading an activity. Make sure he gets the support of the others (be honest with them, do the "you might not like the kid, but I think he'd be good at doing this" speech), but also require that he does things to build up the team that is supporting him.
Use his skills! Have him plan a FLL event, have him teach student how to mentor an FLL team, have him be part of the recruiting effort for new students. If he really has a flare for a technical expertise, find it and use it! One of our youngest students is a CAD wizard. So he is the go-to-guy for that, regardless of age. He has been given the chance to share his skill so he doesn't need to have the "know-it-all" attitude, he can now be the quiet authority.
Good luck! I hope I've helped with my ramblings!
