User blog comment:Menara/The Son of Sobek: The official PJ and Kane Chronicles Crossover/@comment-99.9.229.233-20130417220051

It said in the excerpt Percy looked "about seventeen". Even though that's a guess on Carter's part, I think we can assume he's actually seventeen (otherwise, why would Riordan say it?). That places the crossover timewise just after the events of the Heroes of Olympus.