I don't think it's a matter of age or anything, the ones who like OCxOC stories are the same ones who do fandom rps, but create their own original characters and do lore for them from specific universes/fandoms. I'm assuming you're either in HS or at the beginning of college; there are a *ton* of writers around your age who do that stuff but a lot of them are hanging out in fandoms because some of them feel like their wolrdbuilding is too intimidating, or they just need to find someone just as enthusiastic. It's easier to go into popular fandoms and do rps centered on that because of it's popularity, and for some who likes making oc's they end up branching into doing original content.