I’m not sure if you are asking as a developer or a player. If you’ve seen a game that should be marked as 18+ and is not, please use the report button on the game’s page, and the admins will take care of the rest.
I am a player and recently discovered this platform. I have a blog, in Spanish, in which I talk about videogames and parenting, which is why I am especially interested in the subject of rating by age.