The Film: Harold and Maude (1971)

The Inappropriate: Harold is a disillusioned attention-seeker whose relationship with his mother is poised on the verge of destructively oedipal. Maude is a rebellious OAP. About 60 years divide them when they bed each other. Wrong.
 
How They Make It Work: In Maude, Harold discovers a human being at the other end of life’s spectrum. She’s bursting with vitality and passion. He's not.

Vitally, the flick smashes out pre-conceptions of youth and old age, convincing us that this is actually a very appropriate couple, despite its initial appearance. It works.