"You can't, can you? Even the most precocious students are not deadly with a wand before fifth year; most could not kill until seventh year. But by that time, they've already witnessed, perhaps even suffered from, dangerous and painful accidents that have taught them to treat magic with appropriate respect. Older teenagers can be rash and temperamental, but they do understand the gravity of deadly force. Hogwarts is equipped to handle them. It is not equipped to handle eleven-year-old assassins."