User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4649443-20121130222616/@comment-3146930-20121217001520

It is never stated who is older between Annabeth and Percy. They were both 12 in The Lightning Thief, but it is never said if Annabeth turned 12 earlier that year or if she is turning 13 later that year, like Percy.