I’ll second the votes for The Great Race and The Court Jester. For 11-year olds and adults, you can’t get much funnier than those. My 13 year old son and I can recite almost both scripts back and forth, and we only started when he was 10.
except for one slightly rude moment (easily missed by an 11 year old) I heartily reccomend Galaxy Quest our entire extended family (age range 4 - 43 years old) watched it on Thanksgiving and even my wife and her sister, who despise Sci-Fi and all things Star-Trek, were laughing and enjoyed it.
My cynical media-saturated kid also loved the recent Little Rascals film festival we had.
The Three Stooges forget the movies, stick with the shorts.
Hearty seconds for UHF (it will be the single funniest movie on the face of the earth, if only for the one line that is hollered when the karate-chopping assistants fly out of the closet) and Clue (a little more highbrow but very funy), and if you’re feeling lucky, try out Arsenic and Old Lace.
Ferris Bueller’s Day Off is probably a little mature for an 11-year-old. I loved the movie, but it was only really hysterically funny after I’d been in high school for a while and understood the “angst” of both Ferris and Cameron.
How about Strange Brew? I freaking loved this movie as a youngster and, if I remember correctly, it is pretty clean.
Oh ya, I second or third the Meatballs suggestion!