User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-122.164.234.74-20130211171738/@comment-4547258-20130217153157

Just because Carter said Percy looks 17 doesn't mean he's 17 in the story. Besides SoS being after HoO could spoil the ending of HoO. So it's very clearly safer to have it at the end of PJO.