I know the feeling. I got into spec fic because I read Andre Norton when I was a child, and while some of her books hold up pretty well over time, re-reading them doesn't transport me the way it used to do.
But sometimes it isn't so much that the reader is older, it's that the times themselves have changed, leaving the story, behind, in whole or in part. Case in point—The Stars My Destination by Alfred Bester.
Bester was a pioneer of spec fic and yet somehow I never read his stuff, even though I was reading spec fic as early as (gulp) 1962. So, when I began to write seriously and I heard this work lauded as groundbreaking, I thought I should read it. So several years ago, I did. I hated it.
I don't think the problem was that I was older, I think the problem was it wasn't 1956 anymore. William Gibson says all books are about their own time, and I think that's true in this case. When the protagonist of this book gets angry at the universe because he has been brutalized, he decides to take revenge, and this revenge includes raping a woman who never hurt him. The reader gets to hear his thought process as he consciously decides to do this. It totally and completely turned me against the protagonist and even against the story.
This was not just Bester being a pig. If you read mystery and spy novels from the 50's and 60's, rape was treated very differently than it is today—almost casually, in fact. There were plenty of romance novels where the woman protagonist fell in love with a man who raped her!
But I can't go back, so for me, reading the story for the first time at the cusp between the 21st and 22nd centuries, the stars were definitely not my destination.