I’m surprised by the comments on this thread. I suspect I’ll get absolutely flamed for this but I think it completely depends on the child and their understanding of what’s real / what’s not. And of course the content of the actual film - some horrors are more sexual and violent than others. I grew up watching all sorts of stuff - I watched Nightmare on Elm street when I was about 9 (I’m 40 now) and I loved it, it’s done me no harm at all and I loved scary films - Jaws, Friday the 13th, Evil Dead, tons of stuff. I am the most balanced person I know! The only thing that’s ever traumatised me was actually Watership Down because I love animals and it really upset me!
Following on from this I am with Dh who equally loves horror films and both our dc have seen all sorts of stuff - only if both dh and I have seen it beforehand and know the content. Ds is 9 and we watch all sorts of things with him- poltergeist, the conjuring, the babysitter, Shaun of the dead (although that’s mainly daft, still lots of blood) etc. Dd is 17 and is a well adjusted, sensible teenager who is on her way to university next year.
Both have always known the films aren’t real. They’ve never been scared by anything. Dh collects horror film props and merchandise so maybe that helps.
We just tell them not to say anything to anyone at school etc as people don’t understand.
It was extremely common in the 80s and even 90s for kids to watch horrors etc. I used to go to sleepovers at about 11/12/13 and it would be a treat for all of us to sit and watch a 15 or 18.
I think we are all a bit too over cautious when it comes to this stuff. Not all kids enjoy sitting there watching Frozen or the masses of PG type stuff there is out there. Some kids really enjoy being scared when they know it’s safe - like lots of grown ups too!