The Coen brothers at their very best.
The Big Lebowski is truly one of the greatest movies of our time. Of the three Coen brothers films I've seen (No Country For Old Men, Fargo, and the Big Lebowski), I must say, this is the best.
Besides having a fantastic story with nonstop thought-through dialogue, the movie is packed to the brim with dark, irreverent humor that teens will find incredibly funny.
Yes, the swearing is frequent, the "F" word is said 293 times, but it is not just used to fill up the spaces where interesting conversations should be. It is used more to spice up the dialogue and add a sense of tension to some of the darker scenes. Also, I don't really mind language in my movies, so I suppose viewer's opinions on the language will vary greatly.
As for the sex/nudity, it is relatively mild. There is some full-frontal female nudity, but both times it is obscured by motion and shadows, nothing much to worry about there.
Overall, the Big Lebowski is a brilliant and surprisingly uplifting movie about the "losers" being up against the rest of the world, and trust me, it delivers.
I recommend for 12+, but this also heavily relies on the maturity of the child.