It can be a either or a bit of both. Really depends on the circumstances. Shades are typically created when an enchanter attempts to bind a spirit to their own body in order to gain their power. However this always end up with them slowly going insane as the spirit constantly fights to take control. Some Shades might be made by a spirit just directly possessing them (Theyre fickle that way). and some could possibly be a shade that went through a failed exorcism. Where your in control of your body, but the spirit is still stuck in there, locked away like a voice in your head.
elves would mature at around 20-30 and wouldnt start slowly aging until they hit 300 years old. Most still appearing middle aged even in their late 300s