First of all, this is a great game. The stealth is superb, and the choices you make throughout the game affect the difficulty of the game in later levels, and I think that is what makes Dishonored so fun to play. I rated this game caution for kids under 14 because I think this game can be very violent and graphic, but only if your kid chooses to play that way.
Violence: The violence in this game is very graphic and intense. You can sever limbs and heads, brutally assassinate enemies with your sword, and use various weapons such as crossbows and grenades to dispatch your enemies. You can call up rats to devour a person alive, watching their body be stripped to the bone in a matter of seconds, complete with gurgling sounds. However, notice how I used CAN in all of these descriptions. Your child can choose to run through levels killing everybody, or stealthily sneak around without killing anybody, or a mix of both. It is possible to complete the entire game without killing or being seen by a single person, and you are rewarded with three achievements, but this is very hard to do. (I have completed it, but had to restart many missions to do so) the violence in this game is entirely up to choice, but you will be negatively affected if you kill a lot of people. The game will have a darker ending, and there will be much more enemies you have to face in later levels. I think that for a first run through the game, your child should try to be stealthy, but dispatch enemies as necessary.
Language: Language is not too much of an issue in this game, but still present. Characters will regularly use hell and damn, but you will quickly realize that it is just part of the dialogue. Now, every once in a while, you will be able to hear an enemy use f--- or s--- in a sentence. This does not happen too often, but it is still something that your child should be aware of. I'm not going to go into the whole thing about your child hearing these words at school, because I feel that is a feeble excuse, as they should not be hearing these words at home.
Sex: Common Sense Media has an accurate description of sex/sexual themes in this game. There is one level where your character will have a mission in a brothel. There are no sex scenes or anything like that, but the whole area has the feel of a kind of club.
Drinking: Your character's meeting place is in a bar, and alcohol is consumed, but nothing your child can't handle.
All in all, this is a great game, and I highly recommend it for kids 14 and up. You should talk to your child, and maybe ask them how they think they want to play through the game. If your child has had experience with violent or action/adventure games before, they should be fine. If this is your child's first violent video game, I do not recommend giving them this game, but instead easing them into the genre with a T rated game.
Hope this helped!