I don't even know why the various sections get associated with writing skill, it's something that has been a pet peeve of mine for as long as I've used RP'ing boards. A guy writing a two sentence post in Free can be a better writer than someone who's putting out novella-like posts in Advanced. I view it as more of how much time you're willing to invest in an RP. Advanced obviously demands a larger chunk of time, because you need to go in-depth, whereas Casual or Free is a easier to deal with and work into a busy schedule. Either way, they are categories, not a social ladder - you don't have to work your way up from the depths of Free to "ascend" to Advanced. 

RP at the level you feel like, skill and experience hardly factor into it. I started out RP'ing when I was 13 as well, my first ever RP was in the Casual section (by the Guild's standards), but I realised I prefer longer, more detailed stories so I went into Advanced without giving it much thought. I certainly didn't have the skill most of the participants had (one of them went on to become a published writer), but that didn't stop me from enjoying myself, plus I learned a lot. 

How you behave yourself on an OOC level is another matter entirely. From my experience, most people on the internet don't care about your age, your gender, your sexual preference, your nationality, your skin colour and so forth. It's all about your attitude; if you behave like an asshole people will perceive you as an asshole, if you behave like a chill guy people will perceive you as that (regardless of what you might be IRL). That's the great thing about the internet, you can be whatever you want to be without being weighed down by the burdens inherent in physical communication.