It depends on the book. In each book, his age increases by 1. He starts out 11 in the book 'The Philosopher's Stone' and to the seventh book 'The Deathly Hallows', he's 17.
Harry Potter was one year old in the first chapter. The book then skips ahead nine years and Harry is ten. At the end of the third chapter he turns eleven. To be accepted into Hogwarts School of...