I've been on the site for close to eight and a half years. I believe that's about a year and half or a little longer than the site, itself; and (I may be wrong here) I thought/think August is their birthday. Either way, by the time I'd signed up there were "zillions" of people already here, many of whom seem to still be around under their original user name. It's anybody's guess how many are also here under one or more different user names.
I think, somewhere buried in the forums, maybe the ninth "birthday"/anniversary thread for the site. I think I recall posting on it. I know the 9th isn't coming up (because my 9th is October/November), and I know they were around more - like - between a year and a half and two years when I first signed up. So, if their tenth is coming up before Fall of 2016 that would seem about right.
Just out curiosity I did a search, and it confirms that the site was founded in August, 2006. That backs up the thing that jimmythejock's "nine years" /"over nine-years" would mean he's got his tenth anniversary coming up this year. (I'm operating on a "weird computer set-up", so looking up stuff like forum posts is tedious, but the search and that "over-nine-years" thing make me confident about this answer.