So explicit content is only allowed in private messages anyway, any RP that is done in the forum threads should be fine! I think most people would appreciate that you disclose that you're a minor, to avoid certain inappropriate behavior that they might not think about otherwise. I don't think I need to give you the "be careful on the internet" speech, so I'll leave it at that. 

There's a [url=https://www.roleplayerguild.com/forums/40-tabletop-interest-checks]separate interest check[/url] forum specifically for tabletop games, so you might want to look there and see if any appeal to you. 

I think tabletop stuff is less popular here than purely narrative(owing to dice mechanics requiring the use of outside resources to roll transparently), but there is definitely a following. 

If you give an idea of what kind of stories you'd like to write, or what genres you like to work with, you might find some people who are looking for the same thing. I've seen RP groups form that way.