The Keeper of the Threshold: The Astoria Chronicles

Judah has been frequenting the land of Astoria through a portal in her neighbor's cotton field since she was five. Her daily visits to the centaur Lyle's cottage provide Judah both a friend and an escape from the reality of her mother's mental illness. What Lyle hasn't told her is that Astoria banned human adults ages ago, and the portals of Astoria close upon a human's coming of age. When Judah surprises Lyle by sauntering into Astoria on her thirteenth birthday, Judah soon realizes she has become a fugitive in a land she once considered a refuge.