User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-24205801-20121112034323/@comment-2229042-20121112040559

Hmmm...I think he's fifteen. Percy's 14 in The Titan's Curse, right? Nico is ten. So if Percy is 17, almost eighteen in Mark of Athena, then that would make Nico about three or so years younger, 15, right? So yeah, 14-15 year old range. Cool :)