 The character Kaitlin Cooper left the show after the first season to  go away on boarding school. When she returns almost 18 months later  in season three, Ryan Atwood says that he hasnt seen her since she  was eleven. In the next episode the characters talk about Kaitlin  leaving for boarding school when she was 12. A couple of episodes  later Kaitlin celebrates her 15th birthday. 18 months after leaving  for boarding school.