I would argue that it's your registration date, unless you specifically went to parties and hung out with people on here.
But on the flipside to that, I've been a user (Muzition00) for over 8 years now, but my post count doesn't exactly reflect that. I guess there are plenty of folks like that, though.
while we all know that is true to some extent, there is a longstanding trend of non-participation by newer users that skews these results. there are hundreds of users every month that lurk on tww, but don't actively post, and many of them would fall in the 0-3 years category. the longer you've been on tww, the more likely you are to participate. i mean, i heard about tww in early 2001, didn't register until 2002, and didn't really start posting till 2003
Registered at the end of freshmen year on 5/5/02. Of course was lurking for most of the year since it was kind of hard not to know about this place when they would plaster the free expression tunnel with it