How old was holden when james castle jumped out the window?

The incident with James Castle happened when Holden was attending Elkton Hills, which was a prep school that Holden went to before he was at Pencey Prep, so I'm guessing that Holden was probably around 15 or 16 when he attended school with James Castle. Allie had already died by then (Allie was 11 when he died, Holden was 13).