I think it is either YA or MG, don't walk the line I would say is the hard and fast rule. My caveat would be if it is planned as an expansive series like Harry Potter then walking the line is ok. But never age the hero/heroine too much in the book itself it is frowned on. The line between YA and MG has to do with age but also with content, how the characters are written, how they perceive themselves, what they learn and how they apply what they've learned.
