Percy and Nico are the same age, 19. This is a Harry Potter crossover. Nico, Percy, Thalia, Jason, Leo, Calypso, and Will go to Hogwarts to help fight in the war coming up with Voldemort. The gods were asked to help and they agreed. Chiron picked those seven to go. By the way Percy and Annabeth broke up when Percy came out as gay and Annabeth lesbian. Let's just say Harry gets suspicious. This is in the fifth year but Umbridge is teaching instead of Mad eye moody.