For context, I'm 34, and like many around here a bright teenager. One thing that I really missed was story coherency, the careful assembly of motivations, events, themes, and characters into a cohesive whole. I don't even know how to describe my standards for a good story as a youth, but they're wildly different than they are now, and in particular many of the things I enjoyed then I now consider incoherent nonsensical trash. Things like Star Trek Voyager, which I always found weak, but now I can explain that weakness, or Final Fantasy X, which merely slightly bothered me at the time, which I now realize is because it was so bad it managed to penetrate the thick fog of blissful ignorance I was living in, but only a little. On the other hand, many of the original Star Trek episodes actually make a great deal more sense to me now than they used to. (It's fun to read the Blish novelizations of them; without the campy 60s videography and terrible effects, the true quality of the underlying stories comes out more clearly.)
I woke up to this around 25 or so. It was actually Buffy the Vampire Slayer (the TV series, of course, not the forgettable movie) that tripped this for me; I realized I cared far more about the characters than any campy horror show had any right to make me care, and I began to wonder why, and poke into the mechanics of how that was done. That turned out to be a longish and interesting journey.
(I know I've mentioned several media that aren't "reading" here, but it trivially applies there as well.)
I woke up to this around 25 or so. It was actually Buffy the Vampire Slayer (the TV series, of course, not the forgettable movie) that tripped this for me; I realized I cared far more about the characters than any campy horror show had any right to make me care, and I began to wonder why, and poke into the mechanics of how that was done. That turned out to be a longish and interesting journey.
(I know I've mentioned several media that aren't "reading" here, but it trivially applies there as well.)