You could go by what is considered M-rated for literature in the US. According to Fiction Ratings it is "Not suitable for children or teens below the age of 16 with non-explicit suggestive adult themes, references to some violence, or coarse language.
Fiction M can contain adult language, themes and suggestions. Detailed descriptions of physical interaction of sexual or violent nature is considered Fiction MA."
Personally I would consider graphic depictions of ED as mature given kids are impressionable, and if one on the verge of having an ED (Or already has one.) reads that part of your story it can encourage them, just like how depicting suicidality can encourage a kid who is suicidal. Although how you tag it depends on your story, if it's all mature you could either tag the whole thing, or put up a general age warning and only tag the sensitive or graphic bits. If the story is generally kid friendly with small bits of mature content, I would suggest tagging and putting up a warning for the mature sections.
Overall the nice thing about novels is you can get away with waaay more than comics given it's not a visual medium, so you can be a little more lax with tags if you want the visibility. Like, my comic is totally banned from the app (Desktop only) because is has nudity, profanity, blood and sensitive depictions of abuse and mental illness. The story itself is meant for a 17+/18+ audience, but for visibility I use content warnings for the whole thing, and only tag the super uncomfy stuff. I don't worry too much about a ton of kids seeing it since they don't really use the desktop site, and the warnings at least cover my ass if someone tries to get me in trouble.
Although actually seeing it can be worse than imagining it, at least in certain cases. Mentally visualizing a nude character is less out there than literally having their junk in your face, or imagining a violent scene can be less traumatic that visually experiencing it. At least imo, although triggers are subjective.
Do what you feel works best for your story and readers.