Jurassic Park for me. I had an amazing JP jumper when I was like...maybe 6. It was far too big for me but I loved dinosaurs. Naturally this meant I wanted to watch the film because...well I'm 6 and it's got dinosaurs.
Ultimately I ended up watching it with my Mum and Dad. We got as far as the iconic T-Rex chase scene and I told them to turn it off. Didn't go near the film for another few years.
I've now got my own 6 year old. There's no scenario I could envisage where I even consider letting her watch a film as gory, tense and frightening as JP.
By the time I was 13, I'd watched loads of 18s. Aliens, Predator, Terminator and more. My parents didn't really believe in rating. I honestly don't think it harmed me. My teachers probably worried about me bring this stuff into school, but unless that traumatized other kids, it's fine. Maybe it desensitized me?
For my own kids, I judge it by the kid and movie, not the rating. If it's a movie I don't know, I'm read about it and rating is one of the things I'll look at. I will a read more if it is an 18. My 14y and 9y are pretty resistant, but my 12y is sensitive like my spouse.