Much like parents who trumpet how children should be free to roam and explore without meddling from adults, but never clarify whether they are talking about middle schoolers or toddlers.
I don't think twice about letting my 7 year old watch shows that I steer my 9 year old away from. The 7 year old, a thrill seeker, enjoys things that would give my 9 year old nightmares for a week.
I've never seen it and have no intention ever to do so.
It's more about disturbing atmosphere than gore. Personally I find it interesting in part because it's pretty much based on (early seventies) news reports about crime and serial killers, trying to capture that kind of storytelling.
The horror comes more from socially prevalent suspicions about working class, rural and mentally disabled people than on the nose depictions of violence. A supposedly frightening revolt of the subaltern, of sorts.
I find much of what I see in the news much, much nastier than anything this movie has to offer.