First of all, I've never really agreed with Common Sense. They seem to be uptight about everything. By age 15, a teen should be able to watch what they want. Think about it, 15 and 18 are not that far away. Depends on the child, though.
I am rating this thirteen and up because it is not that bad. The sexual content is not even close to an issue for someone that age. If they don't know what sex is, that is a problem. Try asking your kid questions about what they know about sex. Slowly expose them to things so the can learn about it in a nice, healthy, manner.
As for language, no big deal. My Parents never care about the language in a movies, tv shows, books, etc. Their reasoning: every kid will swear from around age 9 until the day they die. Swearing is a way of expressing emotions, and a child should have examples of people who swear responsibly, and at the appropriate time. Granted, for a thirteen year old, hearing the word s*** is not a big deal.
As for violence, depends on the kid. If there is a game that has violence, or a movie/tv show that is violent, I ask my parents this question. "Do you think this violence will effect me mentally?" They think about this and answer, "No". Then, I ask, " Do you think that watching this will effect my grades?" Usually, the answer is no. Depends on the child in this case. I personally get grades between B+ and A+. My parents know that a tv show won't effect me. Then, I ask them the real question. "What negative effect will occur if I am exposed to this violence?" I let them think about this. Parents, if you can't come up with an answer, your kid is fine. This means that there is nothing to worry about. If you can, discuss it with your kid. If he/she is really interested in this show, let them watch a couple of episodes with you. Watch how it effects them. Do they become more aggressive? Do their grades drop? If so, this is a clear sign that they are not ready to be exposed to bloody violence. If not, they are absolutely fine.