I don't think they had an age intention for KH: CoM. Since it's on GBA, it's also not quite as graphic as can be seen in KHII. Ruthless, maybe, but it's just text. You'd have to use an imagination to see any further, and God forbid that that current generation uses it.
However, it's not quite as deep as KHII, but it could be interpreted as more violent, despite the screen blacking out to Vexen's death, rather than watching all of the remaining members fade into the darkness,
(SPOILER)especially Luxord, who is literally stabbed by the Keyblade.