Just my two cents, as most others have already covered in the comments above mine, but...

18+ doesn't necessarily mean smut and erotica. 
It means it has [i]potential[/i] for it, given that the characters are all adults. 
But it can also mean [i]horrible[/i] violence and gore that people may not want minors involved in or exposed to. 

For example, I might make a sadistic demon type of character who corrupts and possesses, and chooses to wear someone's ribcage as a hat -- I don't really want to be inspiring the next Columbine shooter by exposing a minor to violent ideas. 

Also, some of my RPs have really dark themes -- no one is safe, not even women and children, in my roleplaying. Not a lot of folks under 18 comprehend that, and therefore, I generally keep my roleplays 18+.