Jokes in films and TV you didn’t get until years later.

Nothing film related that I can think of, but there was one joke in Will and Grace that I didn't get until many years later, in the episode "Head Case" when Will is asking why Grace has Double-D Batteries in her bathroom Grace says it's for her Waterpik, then Will remarks that she does not have a Waterpik and then Grace is like "Let's just say that I do". When I saw that episode as a kid I didn't get it, but when I rewatched it as an adult I realized that the implication was that those batteries were for a vibrator.

There's also a crapton of dirty jokes in "Friends" that went way over my head as a kid, far too many to list. Plenty in The Simpsons as well.