In secondary fantasy, I think there is a type of story that can appeal to young adults even though the characters aren't actually the right age. For example, the Hobbit is often shelved as young adult even though Bilbo is certainly not the right age.
Likewise, you might see R.A. Salvatore, Weis and Hickman, Terry Brooks, and David Eddings shelved for younger audiences.
But if you want to call your book YA, the character should be probably 15 to 18 years old. All the others I mentioned would probably write their query letters by simply calling their books fantasy. Then they would let the publishers, libraries, and bookstores figure out the audience.