Well, we know that when Ven left the land of departure he was 16. And we also know he trained with Master Erasqueze for 4 years. So this mean that, the oldest Ven could have been when training with master Xehanort is 12. And, assuming he was training with Xehanort for less than a year, he ABSOLUTE oldest Ventus could have been when he was appointed a dandelion leader was 12 years old.
TWELVE. Thats a lot of responsibility for a 12 year old, especially considering the last union leaders were adults. And taking that into consideration, I guess the other dandelion leaders aren't much older.
I can see Ephemer and Skuld being about 16, and Brain and Luriam being 18-19ish. And if you want to go farther, because Streletzia was SUPPOSED to be a Dandelion leader, I think she is somewhere between 13-15.
It's kinda nuts honestly. A lot of responsibility for kids so young.