i don't know guys, Rikku X-2 seems to be on to something.
This is Kingdom Hearts 2 world, everyone different so that would mean there ages. And yeah Rikku and Paine look older than Yuna they are probly one year older than her, but they probly thought it would be mean to not let Yuna to have some part in the group so probly they made her the leader as the youngest one.

So what I'm saying is that Rikku and Paine are leting Yuna beging leader, even thought she is the youngest.