Ah, nostalgia. A concept I've enjoyed since before I was old enough for it to be reasonably meaningful to me. Partly because I have often associated it with a fondness for or remembrance of times before I was even alive. That is, for example, TV shows or movies that were set in a time before they were made could be nostalgic from the point of view of the writers, who may have been alive during the temporal setting; the stories may even been based on their own personal experiences, in their youth. Therefore, watching such a thing could make me sort of vicariously nostalgic for the time depicted. Or perhaps it's not even necessary, in some cases, for me to feel a sense of nostalgia, per se, but merely to recognize the nostalgic nature of the works.
But I should say, I was born in 1975, and grew up in the 80s and 90s. So generally when I think of nostalgia for my own past, it's for the 80s... but more and more, I find myself feeling nostalgic for the 90s, as well. And I suppose it's possible that eventually, I could be nostalgic for the 00s. One thing I find interesting is that nostalgia, for me, can seem to be triggered by things I remember quite well, but also by things I barely remember, or even things I had forgotten completely. This can be enhanced in any number of ways... I'm sure people who keep diaries, scrapbooks, photo albums, etc., could trigger nostalgia by looking through such things. I don't really have anything like that, exactly. Other people might use home movies, which I also don't have. The point is, nostalgia, in the sense we know it today (as opposed to it's original meaning- see Wikipedia- which now seems almost a bit absurd, to me)... ahem, nostalgia as we know it today, I think, is something that is understandably fairly new to the 20th century (and now the 21st), because we have ways of recording sounds and images, which people couldn't do for most of human history. At least, not with absolute accuracy; of course artists could make drawings or paintings, and such.
But I should also say, I have never related very well to real life. Perhaps this is on account of Asperger's Syndrome, or whatever condition I may have. I've always been touched more deeply by fiction, in literature, television, and movies. So that is mostly what triggers nostalgia for me, now: thinking about the entertainments of my youth. Or revisiting those entertainments, for books can always be reread, and most TV shows and movies can be rewatched, in some form or other. And the internet can be a great help with remembering nostalgic things, including, as I mentioned earlier, things I may have forgotten. I can search websites for lists of old movies and shows, most of which I'll remember, but occasionally a title will strike me. I may not remember any details, but I'll suddenly remember the fact that I had watched it, many years ago. There are also things that I need no reminder that I watched, titles that remain in my memory always... but the details of which elude me. Oddly, my lack of detailed memory of some things may be more intense than my lack of memory for things from much earlier. That is, there are things I remember with greater clarity from when I was a young child than things from when I was a teenager. Either way... regardless of when I saw something, how much I enjoyed it, how well I remember it, etc., memories both clear and fuzzy can trigger nostalgia, and the internet certainly helps with that. Which, I suppose, is why I began writing reviews of things, and why I wish I'd started doing so much, much earlier in life. If I had a time machine, I'd go back and tell myself to take notes on everything I read or watched. It'd probably help me remember it better in the first place, but whether it did or not, at least I'd have valuable reference material in the present, for writing reviews on my website. Meanwhile... I've decided it would be good to start special subpages in a few of my review sections, specifically to deal with the entertainment I find nostalgic. And over time I've added other categories of nostalgia, beyond just entertainment.
(This will all be important if scientists should ever need to create a clone of me, and program it with my childhood memories.)