Dawson Casting: The titular character's sidekicks fall under this even after bumping their age up from the novels. In the book, Annabeth is 13, while in the movie she's 16, and actress Alexandria Daddario was 24 at the time of filming. Similarly, Brandon T. Jackson was 25 also playing a high schooler. This is even more jarring considering that in their most prominent roles coming into the film, they were already playing adults: Daddario in White Collar and Jackson in Tropic Thunder. This trope will be very present in the sequel, which was filmed three years later with the same leads.
Justified in Grover's case, as he is said to look older than the others (he has a goatee in the first book already) and is in fact 24, although since satyrs age half as fast as humans, he is the equivalent of a human 12-year-old.