You must be 13 or older to have an account on NaNoWriMo.org. But all ages are very welcome to take part in National Novel Writing Month, and we encourage younger writers to sign up over at NaNoWriMo's Young Writers Program website. It's a similar challenge for participants 12 and under, as well as those participating as part of a K-12 classroom group.
NaNoWriMo Young Writers Program participants get to choose their own word-count goal for November, and also have access to great noveling workbooks and fun, kid-oriented writing activities. Like NaNoWriMo, membership in the Young Writers Program is free.