Young people engage most with young people.
That’s a problem when writing crime stories for them. It suggests authors need to come up with young yet credible sleuths. Having the characters’ parents around is a bit of a pain as well.
But I’ve had enough of careful justification. If/When I write another crime series set in an alternative Britain (and, yes, it’s in the pipeline), I’m going to refuse to explain why my two main detectives are sixteen years old. I will trust the reader to accept that’s just the way it is.