I agree with the aforementioned relativity aspect. I can think of two calculable ways to look at it. If, as a Gaian, you're half the age of Gaia itself (or more, of course), you can count yourself as an oldie. Or, if you can graph the ages of every single active gaian and you're in the top whatever percent, you could be an oldie that way too. But if we're going to bother being mathematical about it, the first one seems much more applicable.
And I think that for certain parts of the site, that can be different. For example, the forums have been here forever, and I didn't hardly use them when I started (on another account) so I might not be "old" there, but in zOMG I am ooold, even though I didn't start that right away either, or play regularly from the time I started until now. I'm the one going, "I remember when this was broken" or "when there was only one difficulty option for SS (or anything, for that matter)." Oddly, I don't even feel like that was that long ago. So there I feel it, whereas here I have to go "wait, whut?" when other people are reminescing about things I just wasn't around to experience.
And when it comes to Gaia's storyline, I will always be a newbie because I don't read it.
Age can be relative in certain discussions, but not all.