It depends.
Xion's the same age as Sora because the memories sampled were of somebody Sora's age.
If Vanitas was replicated the same way Sora was, then MX would've sampled memories of, for example, Ven, then Vanitas would be Ven's age (and he is). Then you have Xion's absorbing the rest of Sora's memories, thus changing her into Sora. MX could've found a way to prevent that from happening, or for all we know, it could've been happening, we just don't see it.
Anyway, I somewhat support Xehanort being a Replica, the only factor that prevents that from happening is the soul. I'm prety sure Replicas don't have a soul, and they're pretty much duplicated memories attached to an artificial body.