User blog comment:SayuriDarling/The Mark of Athena Official Discussion/@comment-4649443-20130319001420/@comment-3285234-20130402143936

TTC happened near/during the winter solstice. Hence It's the same year as SoM.

Percy was 13 in SoM. But he turned 14 in August. So he is 14 in TTC w/c happened in December.