The US has banned stories containing child sex, and has ruled against certain acts in pornography like anal fisting as being against obscenity laws.
This is the reason, as I understand it, that Elliquiy and other erotica sites don't allow any written word erotica featuring under aged characters.
Elliquiy has rather few legal obligations.
1) We are required to display a warning on the home page of this site that indicates that Elliquiy contains adult content.
2) We are required to report incidents of child pornography. Not drawings, not renderings, just photographs.
3) It is against the law for us to knowingly tolerate the use of our servers for breaking the law. For example, you cannot use Elliquiy's PM system to buy and sell drugs.
4) Similarly, we can't tolerate death threats, etc.
We are not required to ban literature in any shape or form. Elliquiy's limitation is driven by the community, it was 14 for a time and it's 13 on my other sites. There's no legal obligation for me whatsoever to have it. It is simply a personal choice on my part, with a higher limit on Elliquiy proper.
We aren't even required to ban minors from gaining entry. We're just prohibited from initiating the transfer of erotic material. Even then, the actual age limit is 17, not 18.
We go above and beyond as a courtesy to our members and to set a standard. It has nothing to do with fear of prosecution on these grounds.
Red Rose Stories reached an incredibly small audience (dozens), was isolated (no one willing to stand with them), and was run by a woman whose mental health was not capable of seeing her through the legal process, so she pleaded guilty to what was generally understood to be a rather poor case.
In the event of the US government attempting to shut down Elliquiy, we're more than just a blip on the radar. This is a decently visible site, with thousands of visitors. I don't have just Elliquiy to voice my concerns, I have well trusted accounts on other sites like Slashdot that can stir up some defense for free speech directly. I would also be sure to contact other major websites that might similarly be targeted next - ASSTR, Gelbooru, etc. It would catapult Elliquiy's visibility, and I'd dress up in a suit and stand before a judge with a lawyer from the ACLU advising me.
The thing is, prosecutors know this. They go after weak, small sites first, to try and build up case law. In our case, we don't actually physically mail anything erotic to anyone, so Polk County Florida's trick won't even work here.