I tend to personally overtag (everything on my journal is treated as 18+ by default). Less because I worry about minors reading stuff they shouldn't (I used to be a minor in fandom, I know how that stuff works) than because I want to pre-empt overconcerned adults with different moral standards than I have - something I have mostly seen happen along a country of origin adjacent dividing line of conservative US Americans vs. Europeans - complaining about what kind of content that mentions sexuality is appropriate for teens.

I use the 18+ rating not just as a content marker but also and maybe even more importantly as an indicator of the maturity levels I assume when discussions crop up. It's my way of saying: "I assume you know when to bow out of a conversation and how to use a back button."

Which...I hadn't articulated to myself like that before, so thank you for starting the discussion.