Greg's 11 in the first book. 12 in the second. 13 in the third. Was 13 in the begining then turned 14 in the fourth. I think he's either 14 or 15 in the fifth book.
Actually he's 12 in Rodrick Rules and The Last Straw, and 13 in Dog Days and The Ugly Truth. I think he'll turn 14 in A New Life, but that's just me.