User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-122.167.30.102-20130327091725/@comment-4902593-20130327122812

I think Annabeth is younger than Percy by a couple of months. My reason is that, both Percy and Annabeth are 12 in The Lightning Thief, towards the end of the book, Percy becomes 13. If Annabeth's birthday was during the summer, then she would've told Percy about it, and if her birthday was before or after camp, then if you think about it, she would still be younger than Percy. So yeah. Also I think The Demigod Files mentioned that Annabeth was more mature than Percy. That doesn't mean she's older 'cause girls mature faster.