These are my estimates
Vexen: Definitely the oldest. Now, how old? I'd say he's in his late 40s or very early 50s. Besides, he's always complaining.
50, I would estimate.
Xigbar: Might as well. Just younger than Vexen by a few. Probably early or mid-40s. Probably 46.
Lexaeus: 40.
Xaldin: Xaldin will be a 35-36...
Luxord: He definitely looks older after these two, as he doesn't strike me as that young. Late-20s-early-30s. Though I'd go for 29-30..
Saix: His face actually looks young enough, but old enough. So I'd say around 28.
Xemnas: Xehanort looked very young, so I'm guessing he will be on mid-20s, so far. 26, granted.
Marluxia: Now is when we start getting young. He looks to be about...24-25, at most.
Axel/Larxene/Demyx: This three have to be put close together, as they are definitely in the same age range. I call them, The College Group, just for my own weird entertainment. From 19-21, at most. Demyx will be 21 (I don't know, he just look like a 21 to me). Axel and Larxene are probably 19-20 in seriousness, Larxelness aside.
Zexion: He can't be younger than Riku or be the same age, he has to be a year or two older. So, 17-18.
Roxas: 14. Unless he aged in that one year, 15.