Whew, a lot to unpack here. Most roleplays don't shoot down someone for their age unless the RP is 18+ (deals with very adult situations or is a sexual 1x1 thing on PMs/Ims, I dunno). So let's drop the minor thing for now and focus on your real question: "how do I find my interests in Roleplay?" 

You say you only do fanfiction; well, what are your fanfics about? What series, characters, etc. It's pretty obvious that RPGuild loves doing RPs based on anime, though lately many roleplays focus on OCs or original characters in those universes. For example, a My Hero Academia RP focuses on a school that is not UA. 

Second; what makes you want to write those specific fanfictions? Is it the characters? The genre? Are these more action oriented shows/movies/books/media or do you care about the characters? I personally love adventure stories, and while doing an Indiana Jones or an Uncharted based roleplay would be fun, I can take things from those games and movies that I love and turn them into an original roleplay idea.

So, you need to be specific. What fanfics are you working with, and what is it about THAT particular thing that makes you want to write fanfics about it? You get into that, you can find some interests worth writing about.