Both my kids like to read. I know, I’m a lucky parent.
Because they like to read, they do it a fair amount and as with anything you spend time practicing, you get better at reading the more you do it. In turn, being decent readers, they tend to want to read “more grown-up” books — or, at least, more challenging books. Which usually are aimed at a somewhat older audience — and so then you get into questions of content and “appropriateness.”