I'll occasionally check my own register date to see how old the forum is when the date is relevant like adding the forum to a forum directory that requires the date.
I do keep track, I think it's important to share the milestones and get everyone together. I usually host some competitive giveaway or event to celebrate each milestone
I would, but I would only make a big deal of it annually. I would try to do larger events around the anniversary and hype it up a month or two before, but I wouldn't really do it outside of that.
Dang and here I thought Sonic Stadium was the oldest Sonic website. 😛 I think DreadKnux founded The Sonic Stadium Message Board in 1999. I should log into my account over on Sonic Blast again since it's been a while.