@Oliver My friend, you've simply run into ageist jackasses that can't wrap their heads around the fact that age is not a definitive guideline for maturity. I have known 15 and 16 year olds who act far more grown up and level headed than some other people in their late 20s, early 30s.
Never dumb your writing down to make other people "happy", anyone telling you that load of garbage is not worth your time and are not representitive of this community at large. I have roleplayed with a lot of people your age, and the way I see it, if someone is actually putting effort in and are capable of putting together solid character ideas and quality posts, who cares how old you are?
I myself started roleplaying at 14, and I was writing with people who were years older than me, some even in college. They never made me feel unwelcome or awful about my age, and I considered them to be my peers and friends; I wanted to improve my writing because of them. If they treated me like some people are treating you, it might have killed my aspirations to roleplay.
My advice is to find a game apart from the problem members, and if you don't have to mention your age, don't. I had one guy who started roleplaying with me when he was 12. He's 16 now and I only just found out his age this month. It wouldn't have changed my perception of him as a writer; he's always been pretty damn good.
So please don't get discouraged. I personally think it's awesome that people are trying to tell a 13 year old to dumb their writing down; it just means you're breaking down stereotypes and are way ahead of the curve for creative writing.
@Vor I tend to disagree that Advanced takes more time than Free; my previous forum didn't have set standards (it was a fandom forum with a small RP corner) and where it used to be roughly casual standards of doing a few short paragraphs per post, one only had to post every few days.
Then the community at large started gravitating towards short 1-2 sentence speed posts, which are the norm I see every time I look at the Free section, and it was literally a commitment of several hours a night of rapid back and forth posting just to stay relevant to the game. For time commitment alone, approvimately 3 hours a night every night vs. Spending an hour or two on a single post every week or two is not a comparison; when I RPed in Free standards, it absolutely demanded more time than writing for Advanced does for me now.
Of course, not all Free RPs are speed posting games, but even quickly now I can see games that have been around for a few weeks or months and have well over 1,000 posts. How long do you think those players are spending a night just writing?
I'm also just going to flat out say that if someone's writing in Free but is capable of at least Casual standards, they're handicapping themselves if they want to actually practice quality writing and improve as a writer. I'm not saying good writers don't exist in Free, although I have seen some posts in Free that honestly belonged in Advanced but I think people fear an elistist stigma from it or something, the whole short form post doesn't give room for descriptive writing and paragraph structure. How much character development can occur when you're conveying thoughts two sentences at a time? It might take 50 posts just to leave a room.
And there's nothing wrong with that and people enjoy that anything goes, quick turn around roleplay style where it's more like a chatroom roleplay than a collaborative storytelling excercise. Everyone has different wants, and that's great. I just wanted to interject that if there's good writers in Free, they're not practicing it, and from personal experience, Free writing has taken up way more of my time than Advanced has just by posting volume alone.