One of the "classical" SciFi books that I hadn't read yet - fixed it. Ender is 6 years old, and he's seen as the final hope of humans in front of the imminent invasion of aliens (who already wreaked some havoc in the past). The promising children are sent to the war school so that they become soldiers and officers; the school supplies a number of games to teach military strategy to children. Ender climbs the grades very quickly, and we follow his training from his recruitment on. It's quite a nice read, even if it's sometimes hard to put it back in a context of "wait, he's like 7 years old now?".