But it seems like he can change it to some extent. Or maybe he just knows how to make himself older (that's completely speculative but idk how else to explain this phenomenon). Just looking at KH1, it can be inferred that Ansem was able to change the age of Riku's body to fit a more adult form, regardless of what heart(s) composed him. He didn't take Riku's body and then gave it up. He needed it. That's why up until that point he was always hunched over in a brown cloak.
It's canon that another heart in someone changes a person's appearance.
I don't see how ELSE Xemnas could know its location. Eraqus was given the information from his master when he inherited LOD, and gave the information to Aqua when she became a master. So since Terra doesn't know about it and MX doesn't, only Eraqus could, which means that Xemnas derived that information from Eraqus's memories.
Xemnas learned it from either Eraqus or Aqua's LS.