The Witches is written for young people, and it's told from the point-of-view of a young person, so it's not surprising that there's a lot of talk of youth within the story. What's neat about this book, though, is that our protagonist is young, but he's wise beyond his years. For that reason, we get the best of both worlds: observations on youth from a young person (we wouldn't want it any other way) but keen, mature insight into these observations. Most of the characters in The Witches are adults (with some quite elderly adults, at that), but that makes the children, and the theme of youth, stand out even more.
The Narrator would have had a much easier time defeating the witches if those pesky adults didn't get in the way.
Where the stink are all the children in the story? Roald Dahl should have introduced us to more young people instead of making most of the characters adults.