"What matters are the Scribblehub Content Guidelines, which explicitly indicate that 18+ is the age of consent. "
ScribbleHub guidelines - there is a reason they are called guidelines after all and not laws - are arbitrary rules set by ScribbleHub - Americans I presume primarily as the age of consent is 18 and judging by the user base - without any legal binding whatsoever. The only thing they indicate is that the "age of consent" in stories featured on this site is 18 unless you want to be potentially taken down.
But I doubt strongly that the guidelines will be enforced anyway,
Unless you're talking about publishing your novel on Amazon or something, the only law you abide to are the laws of the site you're publishing your story in. In that case, you should abide by the guidelines listed in Scribblehub.
If you fail you fail to comply to them and a reader reports your story, either the chapter will be taken down, or the story will.
The guidelines exist for a very good reason, you can be sure that they will be enforced. The only way your story will get away by breaking the guidelines, is if it stays unnoticed.
And if the related parties aren't human? Must a elf be over 100 or just any +18 elf in their chibi form will do? And the contrary can also be ask if there is a sapient species with a lifespan inferior to 18 years?
What about worlds with diferent years duration than earth?
And about non-biological beings, must a sex droid wait 18 years before enter service?
If the elf is a child at 18 years old and is depicted in a sexual scenario, the story may or may not have action taken against it, I dunno, I'm not a lawyer.
If the species has a life of less than 18 years, I suggest you don't depict them in sexual scenarios because they'll never fit into Scribblehub's guidelines.
If the years are longer than Earth's, make it 18 years in the way that the characters of the story consider as 18 years old I suppose?
A sex droid is not capable of refusing or giving consent, so the question is meaningless.
If you make a fully fledged AI though, then it may be better to wait until it is 18 years old.
And if you feel like any of those situations feel arbitrary, then all I can tell you is that the guidelines were probably worded the way they were for the sake of advertisers, which don't care if your character is an adult at 5 years old, they want consent to be only at 18+, so the site's guidelines have to enforce that.
... Either that or it's because of the American laws, regardless, it's probably something beyond Tony's control.