I recently started reading Anna Karenina, and even for me as an adult, there's a lot of people with a lot of interconnected relationships to keep track of. But I am surprised by how moving I find it - I guess I expected it to be more distant somehow, but the people really spring to life. If I'd read it as a kid, I imagine I would be relating differently to all of these very adult concerns.
One of the best gifts I ever got was when my dad plopped down a big box full of old classical adventure novels (Twenty Thousand Leagues Under the Seas, King Solomon's Mines, Captains Courageous, Three Musketeers, Count of Monte Cristo type of stuff) and I devoured all of them over the course of the next year or so. I'm sure I would appreciate a lot of different things about them if I read them now, but they certainly held up in terms of being engaging in spite of them all being 100+ years old by the time I got my hands on them.
I was a precocious reading kid too, and I sometimes wonder how much I understood of all the stuff I read. I feel like I remember it decently enough, but there must have been a lot going over my head.