This question's been rattling around my head for a while, from back when we were wondering what the web was going to be for (the answer turns out to be Scrabble). So: there are websites where things actually appear, and then websites which link to them, and other websites which link to those aggregator websites, and eventually you find your natural place as regards your interests and available time, with the right list of sites (including just your mates' blogs) that you check for stuff, and of sites (again) that check you for stuff.
For any piece of information, the patterns of distribution look something like a tree: the trunk is the initial source of it, then there's the branches, and the twigs, and eventually the leaves.
Definitions first: You're "hip" on a subject if you are one of the leaves of the tree, if you know enough about the subject to state an opinion, not neccesarily yours, if it comes up in conversation. You're "cool" if you're at least a twig, if people feel they can come to you and ask "What is good to have opinions about, and incidentally what are your opinions on those topics?"*
Obviously these roles change depending on a subject or specialization. Pick one which allows you to answer the question.
[Edit: argh no-one is answering this, ftb I can't explain an idea to save my life. Question 1 = When did people stop asking your opinion on a subject? Question 2 = When did you stop having an opinion on all of a subject?]
* It doesn't have to be "pull" like this, it can be "push" instead, where you tell them what they should have opinions about, as long as they listen :)