It's just not on.
I am only 24 and when I was 10 my parents would let me watch a couple of 12s they had watched and thought I'd be OK with (Titanic, for example and a few comedies that involved swearing). I don't think I was allowed to watch a 15 til I was 12 or 13 and again that was once my Mum had watched it and decided how I'd have reacted. Once I reached 16 my parents let me decide for myself what I could watch but did advise me that something might be disturbing - even last year my Dad told me that The Woman in Black might be a bit jumpy for me, so I didn't watch it. I don't like things that make me jump.
But to let a 10 year old watch an 18 is ridiculous. Now if a parent had watched a 15 and thought OK it's got a few rude words in or one disturbing scene but they thought their 10 year old could handle it I may get on board with it especially if they were willing to watch it with their child and explain anything that needed explaining. But an 18 - there is just no excuse, is there?
14 years ago when I was 10 I used to go to friends houses and their parents let them watch whatever they liked too so it isn't a recent thing at all. On the Hallowe'en of 2001 we watched IT, Chucky and Freddy vs Jason. I actually fell asleep during the latter - slashers have never disturbed me or at least, not since I was old enough to understand them. However, I don't think I was ready to watch things like that. I was quite morbidly interested in gore and supernaturally things that I had seen when at my friends houses.
I can't remember any of them really upsetting me though but that's probably because at 10/11/12 my friends thought slashers were the things that made you brave. If we had watched something more psychological I imagine I'd have been up all night.
I just think using some common sense is in order. I didn't hold it against my parents for not letting me watch them. In turn, they realised how mature I was and thus let me watch things for 2/3 years older than my age because I was mature enough to deal with it.
I think you do what is right for your child.
However, just allowing your child to watch whatever they want because you can't be bothered to monitor it or you don't want grief from them is piss poor parenting.