Probably both. As we are exposed to more of a thing that has varying quality or varying substance we begin to make judgements of "I like this more than that" or "This is better made than that." In most cases our judgement gets better over the years.
But the younger we are, the less depth and realism it takes to entertain us or move us emotionally, because we have less knowledge of the real world and less experience of real life.
In the book An Episode Of Sparrows the nickname for children is "sparrows". In one scene a grownup tells the very young heroine "You're making a mountain out of a molehill." and the girl petulantly replies "A molehill
is a mountain to a sparrow!" And so it is.
As we become less naive, the mountains have to be bigger or more unique to fascinate us, and anything that seems insubstantial quickly loses our attention, unless we have some other reason for being drawn to it.
That applies to both fiction and non-fiction. There's nothing duller than a third grade math book, unless you love math and have just started third grade. And there's nothing more boring than a book about frugality or simple living that has nothing in it except the same 100 things you already learned ten years ago.
PS. And that's all I'm going to say today. Posting in the middle of the night with nobody else online always makes me feel like I'm talking too much!