As the parent of one 9 year old and one 12 year old we usually allow ours to watch 18 movies but ask them to look away if we feel there is going to be gratuitous violence aka severed limbs, stabbings etc. We also tend to feel uncomfortable about blant sex acts as well !!
However we do feel they both understand the difference between films and real life and it does give them an insight into what people do and can do to one another. After all the news often contains far more horrific sights and that
is real which is more disturbing.