It is an interesting question. Certainly, special effects have advanced to the point where movie effects certainly look a lot more realistic now than they did 30 or more years ago. So I can see why young people or people new to horror would look at older films and feel that the effects are unrealistic compared to some more modern films.
However, if you grew up with a certain era of film, these films could continue to have a more profound effect on you than if you were watching them for the first time.
I remember some of the films that really gave me nightmares when I was young - when I watch them now I still get a sense of what I felt back then (although not as strong obviously). I think this is because they are awakening feelings from my childhood, rather than their intrinsic scariness as seen as an adult. But fears are often about things that have affected us in our childhoods.
It is also a bit about expectations. If all you know are movies with "modern day" special effects, then that is what you expect. Those of us who grew up with older films don't have that same expectation. And that isn't necessarily a statement about age - if you grew up watching movies of bygone eras, then your expectations would be different than someone of the same age who only watched contemporary films.
Also, I think it depends on what attitude you carry with you when you watch the film. If you expect that it is going to be an old film with crappy effects, you probably won't be scared. But if you watch with the expectation of being engrossed and "losing yourself" in the film, it is likely to have a different emotional effect.
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