"Kick-Ass" is a comic adaption that is both action-packed and satirical.
It is about a teen that decides to fight criminals and help people - without having any supernatural powers. Basically, the plot is a lot like the one used in the "Batman" movies, a character decides to set a sign that every average person can become a superhero. However, "Kick-Ass" also is a parody.
It features several comical moments and references to comics, movies, etc.
If you compare it to "Superhero Movie", another comic book parody released two years earlier, "Kick-Ass" is cruder, more violent and has a more interesting sense of humour. This is a movie from superhero fans for superhero fans. Fans of the genre will propably find it an excellent movie, others propably won't. It still is entertaining and funny. However, it lacks originality and is not the best superhero movie.
(SPOILERS may follow:)
As for the age restriction, the movie has quite a lot of violence - both stylized and realistic. There are scene featuring gun fights, fist fights and extremely choreographed fights including a sharp weapon. These scenes are bloody, but however, they are handable. Other more graphic scenes include a man being crushed in a car compactor - we see blood splattering onto the window - and a scene towards the ending in which Kick-Ass and Big Daddy are tortured by evil men - they are beaten and set on fire.
A scene towards the beginning shows a man who is threatened to have his finger cut off by a wire-cutter. The impact is off-screen, but we hear screaming.
A man explodes in a giant microwave. There is blood everywhere, but it is meant to be comical.
Some other violence.
There are several sexual references. At the beginning, masturbation is meantioned quite a lot, we see tissues falling down, but we don't see explicit detail.
The language is pretty R-ish.
You have to see that both the violence and the language is mostly done by a 11-year old girl, who enjoys doing this.
(END OF SPOILERS)
Mature teens around 15 should get that the movie is both satirical and fictional, and should not have any problem watching this.