For me, I basically treat 18+ like I do an R rating for a movie: it probably means sex, but it just as likely means content you wouldn't get away with in a PG-13 movie, like harsher language, torture, more graphic deaths, more mature themes like exploring the darker side of humanity to their worst aspects, etc. It also means my partner(s) is probably an adult, and someone I can likely do those things with.
My 1x1 has this tag in case anyone gets curious and reads it, because at its core, it's based on two PG fandoms, but we took the PG limits away so we could better flesh out the world and characters and not hold anything back. It's a courteous, "anyone familiar with these fandoms knows to expect the basics of them, but also to be aware that the characters and situations involved will be more mature than what you're familiar with."
Some of the plot points we have planned are going to put some of the characters through the wringer later, and we've already explored a few aspects that wouldn't fly in the original canons (like, yes, implied sex). There was also a brawl where one character beat the absolute shit out of another (we're talking, "almost broke her rival's neck and held back" levels of violence), has faced zero consequences for it, and won't get any comeuppance. She will, for all intents and purposes, get away with it when in both original canons, that kind of thing would be one, not be nearly that violent (and more likely played for laughs instead of drama), and two, punished eventually.
Does it suck for people under 18 sometimes? Yes, but that's life. There are some things you just need to be patient and grow into. We've all been there.
