Kids are wildly better than adults at most types of learning—most famously, new languages. One reason may be that adults' brains are “full,” in a way. Creating memories relies in part on the destruction of old memories, and recent research finds that adults have high levels of a protein that prevents such forgetting.
Via FastTFriend
More dramatically, their brains could barely weaken their synapses, a process that allows the loss of useless information in favor of more recent data.