I don't know about you, but I personally love Minecraft, Scrolls and the Entire Batman Arkham Series ( Asylum, City, Origins, Origins: Blackgate). So to me as a ## year old it will be hard for me to get Arkham Knight. I totally disagree with ESRB's rating. So you understand this better here you go: What's Batman Arkham Knight doing with an M rating? Check out the gaming week in review - Emerald Media
Not sure if trolling but I'll explain anyway, M for Mature rating. It's about the same as an R rating for movies. You have to be 18+ in order to purchase/rent a copy of the game. ESRB is the "Entertainment Software Rating Board", they give games their rating (E for Everyone, E10+ for Everyone over 10, T for Teens+, M for Mature+ and A for Adult Only, etc). They rate based on factors like amount of violence (gratuitous or not), nudity, language and the like and assign a 'rating' to the game on who the intended audience is based on those factors. An M rated game tends to receive it's rating for 'Gratuitous Violence, Excessive Language, Suggestive Themes' and the like. As for my take on their rating it M, it must be a much more violent game than the last few installments to get such a review. I understand that Arkham has a history of violence given I've played all the games myself, but they listed suggestive themes and blood as well... It's possible there is much more blood, as well as some very either violent or sexual themes that go well beyond the previous games. I'm not too concerned as I'm 22 and can get the game either way, but higher ratings don't get earned without reason.
The Arkham series is my favourite game series ever. Personally I don't agree with the "M" rating. I'm still going to play it regardless, but it's still silly.