I was also hanging out with friends at that age, going ice skating, going for lunch, the cinema, etc.
But watching 18 rated movies at 11? I would never in a month of sundays let my kids do that. Some kids start their periods at 9, doesn’t mean they should be watching 18 rated horror movies. I was never allowed to watch that kind of stuff at that age, I also started at 11, and never did behind my parents back.
At 4 or 5 I watched a horror movie at my grandmothers and honestly have been scared of the things in it ever since. If you Google the physiological effects of kids watching horror movies you’ll see it can make them have anxiety, sleep issues, etc.