User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-24205801-20121112034323/@comment-3146930-20121112051159

Percy is actually 16, turning 17 in about a month during The Mark of Athena. Percy would be about 14 during The Titan's Curse as his birthday was after the events of The Sea of Monsters, and at this time Nico was 10. So based on that, if Percy was 16 in December (during the events of The Lost Hero), Nico would be 12. So if Percy is turning 17, Nico would be 13 by the end of the year, as we don't know exactly when his B-day is because of his time in the Lotus hotel.