You notice because movies, TV, friends, family all give a glimpse of what is the "norm". When I watch a movie, I always look at cars in the background to get a good idea of when it was taken. Not time piece movies, but real time movies. You can tell by looking at someone with a mullet that it was probably in the 80s. You can easily spot a chick with 80s hair. When you see neon clothes or Miami Vice type wear, you know it is the 80s as well.
The same applies to any other aspect of pop culture. Nothing new here, people for the most part are taught threw conversations, history, television etc what was popular and when. I am curious to know if you think about what is hot today or the norm will be noticeable 10 or 20 years from now. Are you aware of what is what today? I don't think it is important, because back in the 90s, or 80s etc, you probably weren't thinking what will be the next trend, or what will be outdated. I'm sure we will laugh at blogs someday, laugh at Iphones, laugh at GPS units etc. I just don't think anyone notices that type of thing when you are in the now. Not that they should, but I just find it interesting.