I was around seven years old when I started buying my own Superman comics, but I must've known about him since birth. As llitte kids in summer, in our swimming trunks (I'm sure mine were red), we would all tie our beach towels around our necks and jump through the sprinkler, pretending to be Superman.
Is there a significant number of people who only discover Superman later in life? I don't know. If that was the case, I could see why the spell of Superman doesn't hold for them. But for most people, if you have a fantasy when you're very little, that fantasy holds strong through the rest of your life, even though you may have outgrown it and understand that it's not very realistc. Indulging in the fantasy is a way to get back to being that kid you used to be. So there's some kind of high we get from returning to that fantasy.
The only time this doesn't work--it seems to me--is when you're going through adolescence--at least for some teens, in some circumstances. When we're teens, we're going through a phase in our life where we become disillusioned. We may even become angry if we find that those things we used to think as children are not true. We might be so upset that we need to vent our rage about all this BS. And parents have to be enormously patient when they see their lovely children acting like monsters.
But most of us survive our adolescent revolutions and, when we're in our twenties or thirties, we take back the childish things we threw away as teens. And we love them all the more.