I don't think there's a universal criteria;
If it's in a forum, it's probably the content of what you type. Poor grammer or logic, people won't take you seriously, regardless of your rating or age.
In game:
If you're offering me post game analysis and you're 400 pts lower than I am, I probably won't take you seriously. So I suppose ratings matter there.
I was wondering what most people's critea for being taken seriously in the forums is. I know that a lot of trolls aren't really taken seriously as they are just fun to watch unfold, but for a chess based thread do you look at rating...number of games...age?
What is it that gives people credibility online with you?