http://www.gameinformer.com/b/news/archive/2015/02/24/batman-arkham-knight-tagged-with-m-for-mature-rating-according-to-rocksteady.aspx
While you won’t be able to find the rating on the Entertainment Software Ratings Board (ESRB) website yet, we know that Batman: Arkham Knight has received an “M for Mature” rating. Previous titles in the series (Arkham Asylum, Arkham City, Arkham Origins, and Arkham Origins: Blackgate) received “T for Teen” ratings.
The news comes by way of an IGN interview with Rocksteady founder Sefton Hill. Hill says that the team didn’t set out to achieve a specific rating. ”We never did that in the previous two games... We just felt that this is the story that we really wanted to tell,” he says.
Hill navigated the ratings issue earlier in the game’s development. Conversations were had with both Warner Bros. and DC in order to set the direction for the game, which features a mysterious figure known as the Arkham Knight.