I also watched Evil Dead, Texas Chainsaw Masacre, The Exorcist, Hills Have Eyes, Children of the Corn plus many, many more horror films when I was younger.
I started off by watching Carrie and Christine at around 9, then others at around 11 or 12. I absolutely loved them and completely understood that they were fiction but I just loved that feeling of being scared for an hour or so, safe in the knowledge that it was just a film.
Nightmare on Elm Street and the TV series of Salem's Lot probably scared me the most at the time.
I was, and still am, a massive fan of all types of horror films, and whilst I was exposed to them at a young age, I feel that I wasn't adversely affected by them or traumatised. That's not to say they are appropriate viewing for the majority of children, however. Ratings are there for a reason, to protect the majority, but there will be some children who are natural horror buffs, like I was. I was also quite independent for my age, resilient and was brought up in a very academic, arty family where my siblings and I had a lot of freedom. We all turned out fine. However, I used to teach and hearing that a child had watched the films I watched at their age, would be a safeguarding issue now and rightly so.
True, some images have 'stayed with me' over the years but only in the same way that I recall certain scenes from ET or Back to the Future, for example.
Of course, young children should not watch 18 rated films but not every child who does will be affected. It's best to not allow just to be the safe side.