Interesting to note that the first Die Hard movie is 1988. Just watched it the other day, and it doesn't feel stale at all. Lethal Weapon is 1987, and I bet a lot of kids would recognize "I'm getting too old for this shit."
This is actually a pretty good point, indirectly. The rise of widespread internet culture and meme might have something to do with this feeling. Kids are brought up with international internet lingo and jokes that are ubiquitous and timeless. You don't need to know that joke X comes from old movie Y, you just know joke X and you know it's funny. It permeates your culture regardless of your age, and that creates connection between past and present generations.
No, Seinfield was unfunny to me even when it first aired, and it's not because of what it says.
I can watch 20 other sitcoms from 10 and 15 years before Seinfield (from MASH to Mork and Mindy to Taxi), that I wasn't even adult when they first run, and they are still funny, even when their styles and jokes have been copied....
You might be reading too much into the name. Seinfeld is an example. The point is that some works, due to their popularity, become so embedded in the genre, that they no longer seem as original as they were.
