I think every MMORPG I've played has had this stuff in it. It's always been a thing. As the game grew, the bulk of this crowd moved over to this game. Because of course they want to do it in the most popular games. And really before you say mods, most games can be modded tbh.

18+ is a common thing to put just for maturity reasons. Many FCs put it despite not doing this sort of stuff. So it's hard to really police it and the last time they did, it resulted in a thread with over 100 pages of complaints.

As long as they make sure that people under 18 don't participate, I believe they are allowed to do it.

Realistically, I have not normally met people that young. Usually people that young are being watched by parents who also play the game tbh. But the free trial used to only go below level 50 whereas now you can play up to Stormbloood for free, so it's more realistic that someone that young could play without a parent knowing. Still, there are so many other online games now that they often get into those instead.