I'm fascinated by the universal practice in moviedom of pairing older actors with much younger actresses. Sometimes, this age gap is referenced in the movie itself, or made into the central conflict, but most of the time it's ignored within the cinematic universe.
If I had to pick one performer that most epitomized this practice, Grace Kelly would be the ideal mascot. In her brief, yet memorable 5 year career, she made 11 movies, before retiring at 26 to take up Princess duties in Monaco. Of those, only two (Dial M for Murder and The Swan) feature love interests who are less that 10 years older than her. Three if we count Mogambo, where Kelly (age 23 at the time) cheats on husband Donald Sinden (age 29) with Clark Gable (age 52).
As for the rest, she's paired with old hands. And while issues like class, economics, and legalities might be brought up by her partner as potential relationship roadbumps, I don't think age is ever mentioned in her movies. Not much of a biggie when her co-star is the 11-year-older William Holden, but slightly more perturbing when it's 50-somethings like Gary Cooper, Bing Crosby and Gable.