I guess this should be placed, because I read the terms and agreements and kids as young as 13 can join, but I think that the "sexual content warning" is there to do exactly what you said— well, not exactly but you get the idea. The warnings are there so as to deter young audiences from viewing them. This also allows each of the stories to be equally available to everyone. I think that if a kid goes on to read a story, they'll probably at least remember to read the synopsis. Usually, these synopsis give off the vibe about "those things". And if the chapters contain R-18 scenes, then I've seen that authors put (Chapter Title) [R-18] in their chapter titles...this is also a big deterrent and warning. If people don't even read the warnings down there or the chapter titles, then what can we do? So it all comes down to having the warnings. Those warnings describe what's gonna happen in the story. They are also tagged with "smut", "r18", etc. which also gives off another clue. Using this system, all stories get an equal chance to be seen. Those are my thoughts.
This is a bit of an issue though, as far as I see it. "Kids as young as 13" can
join as members, but you don't need to be a member in order to read.
Personally, I am currently writing a story that is probably appropriate for age groups 13+. However, I put an R-15 label on it because that's honestly the closest thing to the kind of age warning that should be put on my content. So, it really would be good to have a more thorough age-rating system.
The typical standard is...
"All ages" - Contains no offensive content of a violent or sexual nature, no swearing or crass language of any kind (none of the "4 letter words" at all) and no upsetting themes.
Ages 7+ - May contain some physical violence. May deal with some difficult or upsetting themes not of a sexual nature. May contain a low amount of 4 letter word swears, but not the "7 words you can't say on TV" and when they are used it is used casually rather than as part of a brutal tear-down. Can contain character death, but only if it is worked in as a theme of a story that aims to deal with the subject of death. (death cannot be treated casually)
Ages 13+ - May contain glorified violence as well as blood, but not gore and excessive disfigurement. May contain some very difficult themes of interpersonal nature. Can acknowledge or mention the existence of sex and genetailia, but not depict it or dwell on the subject. Can contain more casual character death, but not graphically depict the mangled bodies.
Ages 15+ - May contain excessive mentions of sex and have a heavy involvement of sexual or sexualized themes, but can never graphically describe the act itself. The closest R-15 can get to depictions of sex is mentioning that a character hears it happening in the next room. This is only for intercourse though. It can depict foreplay such as kissing and fondling so long as the details do not become too graphic or linger too long on them and cross into "soft-core porn" territory.
Most of what separates 13+ and 15+ is in the realm of sexual content, but there is a bit of a line on gore as well that is harder to exactly define. Suffice it to say, R-15 allows for a far more brutal depiction of the twisted and mangled gory bodies. You have to kinda go overboard on the gore content in order to warrant an R-15 on the gore alone without sexual content.
Ages 18+ - Do I even have to say this one? The main thing that separates R-15 from R-18 is that R-18 can contain something that would be straight-up erotica with graphic depictions of sex.
In older times, there might be a distinction on the gore content as well. However, due to a cultural desensitization to violence it actually requires skill and dedication in order to cross the R-18 line on gore alone without sexual content these days, at least in a written format (it would be a lot easier to cross the line with visual media.) Simply describing the twisted and mangled bodies with medically accurate depictions is not enough. You would also have to add a psychological aspect to the gory content that causes it to be deeply traumatizing in nature.