Well, I registered here in July '06, which after doing some quick math means that I was 14 at the time. Looking back at my posts this becomes rather evident, because while I didn't post often, when I did I spoke with that cringe worthy tone of someone trying to desperately fit in with the cool older kids. For the longest time I only posted in and kept up with the World of Warcraft thread, and I stopped coming here frequently when that thread began to die.
I feel that the QC forums had a substantial impact on my life, probably most notably on my humour and my music tastes. My tastes developed more in one afternoon of lurking everyone's profiles in the Last.fm thread than the rest of my life up to that point. That's probably the very definition of impressionable young teenager, but I don't regret it for a second.
I still come back here to lurk every now and again, seeing if I can still recognize any of the usernames.