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Ok guys, I'm truly on my organization 13 crack bandwagon today. (Maybe its cause I look up too much art on DA) Yea soo bare with the fangirl on sugar!
Xemnas:He seems to be around his late 20's, early 30's. I think Xemnas is one of the more mature ones in the organization, he maintains the castle, making sure that Axel and Xigbar don't destroy the castle.
Xigbar: Late 30's. Geez...he's practically acts like a 18 year old.
Xaldin: Late 30's but he's more mature than the whole group.
Vexen: He's old enough to get his senior id. HA! Quite Mature
Lexeaus: 30's, I can't tell with this guy.
Zexion: 17 or 18 but he's quite mature for his age.
Saix: Late 20's, early 30's. Hmmm...I can tell that the higher ranked one's (cept Xigbar) are more mature than the group.
Axel: He's either 18 or 20 (not too much younger or I'll feel like a ***** I sorta already am) His maturity level fits his age, he likes to play pranks on the other members and cause trouble and loves getting drunk no less.
Demyx: He's at least 16 or 18...besides Roxas he's the youngest in the organization(in my mind at least) He's extremely innocent and acts like a kid but thats what makes him adorable and soo the uke.
Luxord: He's in his late 20's, early 30's. He can act mature most of the time but he'll act immature when he's around Xigbar.
Marluxia: I think he's in his early 20's. Quite mature for his age but loses it when he see's flowers.
Larxene: She's in her late 20's. Can be quite mature and childish at certain times depending on the situation and her mood.
Roxas: Well he's 15, I think we all know that, he's quite mature for his age, he's the one who trying to stop Axel from doing stupid things.

What I see Marluxia a bit younger!