I always see forums and whatnot as basically a cheap replacement for sitting somewhere and having a chat with people. As such I find it perfeckly natural that people should type to represent the way they speak, if they wanna.
Also the idea that in my downtime and day-to-day life I should be a bastion and beacon of all that is correct and proppa in the English language just in case an ESL learner get's the wrong idea, is frankly scary. I think it's best to provide students with the tools to judge when it's appropriate to use different forms of language, it's certainly easier than trying to stop people using them anyway.
I agree with stil as well, conducting yourself with respect towards other people is ultimately the key. I think there's a bigger problem here that's got nothing to do with "old" or "young" and more to do with "home" and "away". I mean those people that come here and for some reason act in a blatantly disrespectful way to the people and culture here, in a way that they probably wouldn't back home. Maybe when you're faced with a language you can't speak and a culture you can't (or in some cases, don't even want to try to) understand, then it's easy to dehumanise people and begin to act inappropriately. Whatever the reason, it's still inexcusable.