He's 13 in the first. 14 in the second. 15 in the third. Either 15 or 16 in the fourth. And 16 or 17 in the fifth.
Nah, he's 15 in Diary of a Wimpy Kid, 16 in Rodrick Rules and The Last Straw, and 17 in Dog Days and The Ugly Truth. He'll be 18 by A New Life.