MG isn't JUST a length and a character age, it's a style of writing. It's generally simpler than YA, certainly has less mature themes, etc. Do you really think a 13-year-old would be all that independent? Or that you couldn't just gloss over the character age, given that "years" won't mean the same where your story is set? ETA: If you're going to try to write MG (or YA) you really need to read some of what's already out there, just to get an idea of what the market expects.