True, but I'm thinking that different incarnations means something more than JUST different ages.
It's known that something very drastic happened to MX to create both Ansem SoD and Xemnas. So, my guess is that something happened to YX, around the time he left Destiny Islands, which drastically changed him (such as totally succumbing to darkness). For the purpose of my example, let's just say YX was 15 when he left the islands. This new version of Xehanort (the one that succumbed to darkness) grew to become the MX we all know and hate.
The reason, then, that MX would use this YX as an incarnation to fight as one of the 13 darknesses is because it is at this point in that incarnation of his life that he is the oldest, most capable fighter, BEFORE undergoing a drastic change.
He cannot use a Xehnort at age 30 because that is the same incarnation of Xehanort at his current age. He cannot use a Xehanort at age 5, because that is the same incarnation as the YX (age 15) already in use.