Personally I don't think it matters, but it might be the fact that (for games at least) designs might have improved over the course of a few years. I find it very hard to get sucked in by nostalgia (there are some exceptions - fond memories of Pokemon, for example), so I tend to see improvements or dips for what they are.
Another thing might be that people liked the stuff when they were younger and after growing up and their opinions change, they might realise this, that or the other wasn't as good as they thought it was back then. However, this isn't tied to the age of the thing in question.
That said, some things are just plain timeless and have an appeal no matter how old they might be.
Basically, the whole "It's too old, you wouldn't like it" argument is a very stupid one.
Another thing might be that people liked the stuff when they were younger and after growing up and their opinions change, they might realise this, that or the other wasn't as good as they thought it was back then. However, this isn't tied to the age of the thing in question.
That said, some things are just plain timeless and have an appeal no matter how old they might be.
Basically, the whole "It's too old, you wouldn't like it" argument is a very stupid one.