Dumb & Dumber is a surprisingly good film. It includes the typical immature humor of Jim Carrey, but it works in this movie.
Violence: 2/5-Not an issue, just some comical slapstick.
Sex: 2/5-Not a big issue, just some innuendos and moderate sex references, and some brief skimpy outfits towards the end.
Profanity: 2/5-Not a big issue either, with a few uses of "s--t" being the worst it gets.
Drinking, drugs and smoking: 2/5-Not a huge issue, just a few pill popping scenes, but it would probably fly over kids' heads.
Overall, Dumb & Dumber is a fun film, especially for young audiences. Let your "tweens" watch it while it's still funny to them, because they won't be laughing at it by high school (most likely). 12+ is fine, and younger audiences would need an adult with them.