While I'd be considered a "junior" by a lot of these people you are referring to (since I'm fairly new to this site), I want to emphasize before I give this post that I'm technically a "senior" to a lot of them, since I've been writing and role-playing longer (admittedly very on and off, and under different circumstances and mediums to a lot of regulars here) than most (not all) of the people I know on this site. I've definitely been storytelling in general significantly longer than anyone I know on here.

I'm probably going to say some things that will hurt people's egos here, but frankly I don't make a habit out of giving a shit what people think anyway; those who take offense to this can get over themselves.

My first bit of advice for you to say is "fuck them" when it comes to the people who are telling you to hold yourself back. Honestly, so long as you can handle the emotional weight of people in the higher tiers potentially criticizing your work or expecting a lot from you (constructive criticism, that is, since if they're just giving criticism because they feel like it then they're assholes who aren't worth role-playing with anyway), then there's no reason for you to stay in the lower tiers if your writing is on a decent level. A person's age, and how long they've been writing, really don't mean shit, and neither does their education or any "qualifications" they might have to flaunt around like prestige.

You're definitely right for assuming that there will be long-standing members on the site who have extremely inflated egos, because, unfortunately, these types of people (long-standing members, big names essentially) are often full of themselves regardless of what type of group you join, and this doesn't just apply to the role-playing world.

I'm a high level player of competitive fighting games, but I've neither competed in tournaments nor am I a known member of some prestigious internet community, so whenever I join a forum looking for partners to have some good matches against I always end up having them talk down to me, which I am never willing to tolerate. In every instance of this happening, the people on those sites who actually were willing to get off their high horses and accept my challenges ended up getting thoroughly annihilated by me, and this includes people who are consistent tournament finalists/winners. This problem you're experiencing exists everywhere, and while it is true that I am older than many of the people who thought I was their "junior", my age is not the important factor to why they lost to me; I put very little time into practicing and subsequently I'm really only playing at my natural base level, without giving any real effort to develop my skills.

I'm obviously not insinuating that they're all like this, since I'm an advanced writer/role-player myself and I consider myself very open to fresh blood that shows talent and determination for this art, but you will find that what you're experiencing is unavoidable no matter where you go; it isn't unique to this site.

The very first role-playing forum I ever joined had (with only a small noteworthy exception I can think of) an incredibly pretentious staff and a lot of lackeys that followed their mentality. They looked down on other members, particularly newcomers, and pretty much just criticized anything they didn't like (it didn't even have anything to do with quality, they were more or less just bashing people for not catering to their personal tastes and preferences) rather immaturely, despite the rules of the original creator of the forum forbidding this sort of behaviour. Most of their time was devoted to bashing people's character profiles (just because they didn't meet their personal preferences) and writing rather painfully embarrassing out of character fan-fiction in the freeform area that they thought was gold. I was already a high level storyteller at this point, who simply had no experience in text-based writing yet, but I was immediately isolated by some of the staff and long-standing members based on personal/preference clashing alone; their actions here are not the mark of a good GM or role-player.

I ended up calling out the staff (and their lackeys) on all of their bullshit, and when I pointed out how they were breaking rules they actually went and removed rules (the creator of the forum was gone by then) so they could ignore my argument. In the end I publically admonished the staff and left that forum, resulting in a domino effect that eventually killed that site completely. Eventually only the decent people were left on the site, so I came back to it, and even ended up on the staff myself, but that's beside the point. This should illustrate to you that this sort of thing is common. The only difference is that site I joined was an entirely advanced role-playing forum, whereas this site has a huge variety of people at different skill levels and experiences. The problem nevertheless exists; a lot of new people are awkward and just want to get involved, and a lot of long-standing people are wankers who need to get over themselves.

I'm not referring to any particular members here (and there's exceptions to a lot of rules), and I don't know who you've dealt with, so this is more of a general observation of communities (and subsequently anyone who takes this personally can get a life). My suggestion to you is to do what you want. If you want to stay casual, do that; if you want to go into advanced, give it a go. Send me a PM sometime if you'd like someone to talk with. I'm not role-playing myself currently and I might not be for a while (for multiple reasons), but I don't want you to think you're alone here. Never dumb yourself down; I sure as hell don't and a lot of people bitch about the walls of text I toss at everyone. Regardless of which forum you choose to write in, give it your all. No one should tell you who you should be; only you decide that, online and offline.

All the best.