They literally didn't know or particularly care what I was doing from about the age of 9 onwards. Sad eh.
With my own kids, aside from actual porn (which I don't think anyone should watch at any age, but that's another debate), I base it more on what I think will upset or scare them than some arbitrary limit. e.g. I wouldn't have had an issue with my eldest watching horror etc. at 12 BUT it clearly still properly scared her/gave her nightmares at that age. She's 14 now and I have no issue with her watching/playing whatever she wants to watch or play.
For her own part these days, she's not bothered by horror/gore/violence any more (she went through an anime/Korean Zombies phase and like all of us is thoroughly desensitized
) but she is very uncomfortable with even minor "romantic" scenes. And I mean to an extreme ... she was cringing her head off watching "Elemental" in the cinema with us, becuase there were kissing scenes, hahaha.
TBF I don't know if she's bothered by them in general, or just embarrassed/cringing at them when parents are around... which would be pretty normal for a teen.