There's always Indiana Jones and the Last Crusade; Sean Connery was born in 1930 and Harrison Ford in 1942, but thanks to the white beard, Connery looked old enough to play Ford's father.
The Graduate is also a lot less shocking when you consider that Anne Bancroft who played Mrs. Robinson was only 6 years older than Dustin Hoffman, who was 30 or nearly so when they shot the movie.
I haven't seen Heartland so I don't know about that daughter/girlfriend dynamic, but there are plenty of examples both in fiction and in real life, where the second or Nth wife is very much younger than the first.
The most extreme case I know of is Bernie Ecclestone, who through his eldest child, who is in her 60s, is a great-grandfather. Yet he became a father again last year, at 88. So his eldest child could be the grandmother of his youngest. I guess when you're a billionaire...