User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-122.167.30.102-20130327091725/@comment-3146930-20130327102810

It is never said who is older between Annabeth and Percy. They are both twelve in The Lightning Thief, but it is never said if Annabeth's birthday is before Percy's, after Percy's, or even if her birthday was earlier in the year (making her almost a year younger). With the exception of Annabeth who's age is unknown, Percy is the oldest.