The FPS genre is all about players using guns to shoot things, this leads to a ton of blood and violence as is. So, it's hard to find many popular FPS games that are rated by ESRB to be suitable for younger audiences. Most modern FPS games, be it behemoths like Call of Duty and Battlefield or games more focused on storytelling and narrative like Spec Ops: The Line, and all have Mature (M) 17+ ESRB ratings.