I'll get to the point by saying that the series is not nearly as bad as most of these reviews may make you assume. It's more than safe for kids 13 and up, and, in fact, is likely written specifically for this age group.
I'll start with what most people on here make the biggest deal about; the sexual content. Honestly, most of the content is from the first episode of the series, which features a practically naked witch, and some innuendo-jokes. Most of the other episodes only have minor jokes about this subject. (As for the scene with a character pulling up another character's shirt, there is absolutely nothing sexual about the scene. No references are made at all to strip[ping or anything like that during said scene. In fact, it's a very short scene, meaning that if you must, for some reason, skip it, you could simply cover eyes.) The violence is much more prominent, although it's not worse than what kids usually see in their war video games. Basically, there's alot of Blood; not to worry, though, as nobody ever really dies. They recover. The language is simply things that middle-schoolers hear every day, such as a**, da**, or bast***.
The series is very reliant on teamwork, though, and Maka is very good role model, as she is very mentally strong, and fights for what she believes in.
Totally safe for any 13 year old!