Ok, so we all know Ienzo looks like he is 9. And Zexion is like 19 or 20. And we also know nobodys don't age. So my theory is that when you do become a nobody you age a bit. That would also explain why Braig looks 5-15 years younger than Xigbar. Also IF Terra did become Xehanort that means he aged about 7 years when he became a nobody. I say that because it was confirmed that Xemnas was a little over 30 and here:YouTube - Kingdom Hearts II Final Mix Cutscenes part 2 at 21 sec it says sora is 10 years to young to weild a keyblade that means that terra was most likely 24 in BbS.