This is very interesting. It is also my impression that norms for what content is appropriate at what age are different between US and Europe (with US being stricter).

Hhhm maturity levels for discussion is a good point. I think I am going to make more clear what the 18+ stands for in my profile. Because I rarely post anything nsfw but I agree, I expect a certain level of maturity of my commenters if I do a question or meta post.

It always helps me figure out my thoughts when I write things down, so all the answers here have helped me understand the ratings issue better.