They say in SL that two months from now, you won't recognise yourself.
It's true, in a sense. Something about SL makes time accelerate. Things that would take years can occur within months or even days.
The atmosphere is rapid and constant change.
Two months ago from today, I had black colored Linden eyes, wore heels and a Swiss Miss dress, and generally kept to myself with a few close friends from around when I started, and was a rather lazy but constant builder.
Now, today, I wear a pair of eyes a friend made and gave to me, I wear slippers I copied prim by prim from another friend who left in July, I ditched Swiss Miss for a kimono, and increasingly I find I'm not really building anything. There was a short stint in the past week where I tried a different hair style, but it was annoying and wasn't me, so it didn't stay.
And really, that's only counting physical appearance. Who knows how I've changed mentally and emotionally? Can you measure yourself on those without being subjective? Can you rely on other's assessments on that?
I look at old snapshots I've taken over the nine months since I started, and in the very earliest ones, I can hardly recognise myself. Sometimes the people in them haven't been on for months, probably they quit, yet they live on in SL in my snapshots. The eerie part is that those in my current shots will probably be gone sometime, and then I'll reminisce about them later too, when I know them well know. And I'll also probably look totally differently again. What a cycle!
We don't age in SL. But our appearance, as noted above, does shift and how it shifts can be a good indicator of how old/acclimated one is to SL.
One of the first I noticed was with my good friend Doug. We both towered over residents much older than ourselves, and we came to the conclusion that the taller you were, the longer you had been playing. Why? Because LL, for whatever reason, sets the default avatars to be giants. Obviously, if you spent any time at all ingame, you adjusted your own avatar, and more likely than not, reduced height (unless you intended to role play as an Amazon). It's not always true, of course, but it's common enough to be a reliable guesstimate.
Hair and Skin are another factor. Is this person dedicated enough to buy their own skin/prim hair? If they have it, they aren't likely to be brand new. I'd wager about a few months old at least. The quality itself isn't taken into account, because people being people, have wildly varying tastes. You can only class it very broadly, perhaps obviously cheap looking and not so cheap looking.
Profiles are another clue, albeit obvious via the rez date given on the front page of it. Is their profile filled out? Does it have picks? A photo of themselves inserted? How detailed is it? All this goes a long way to guessing the length of time a person has spent inworld. If they've filled out every cranny, they're probably regulars.
The basic hypothesis we could establish here is that the more detailed or complicated looking the avatar and its profile, the more likely the person has spent significant time inworld.
Notice I specified 'inworld' as opposed to just existing. There are accounts created ages ago that look like they're fresh off the Orientation because they logged in once in 2005, and then didn't log in again until two years later to see if they still have an account. In this case, we can see that despite the appearance of a long life through their rez day, they actually are only two days old in inworld time.
We can easily prove this false, however. Some avatars with much time inworld decide to keep their somewhat noobish appearance, whether out of pride or sloth. I myself did not get a 'unique' skin until about six or seven months into it. I didn't have 'flexi' hair until three weeks ago. And that's just using me as a singular example. There are probably more. So, we should take the above observations as benchmarks, or better yet as guidelines there to help us judge a rough estimate, and not solidly nail down an age.
Speaking of appearances and while we're on this topic, I find it entertaining that my avatar itself is built by committee. Lots of the things I wear I got from somebody as a gift or a token, and I feel like wearing them keeps their memory. I wear earrings made by Hazel, a Daisy made by Vlad/Vladia, eyes by Chaos, Slippers given to me by James, a necklace given via a group to me by either Hazel or Lillibeth, flexi hair that Madison encouraged me to find, a tattoo from a vendor at Pero who I happened to meet one day, and a skin given to me by Jurin in her ungodly large freebie box. There's probably more I'm forgetting about.
And yes, I will admit I'm bragging about this. Whenever at look at my attachments, I think of these people, and how I got to count them as friends.
By the way, this isn't to say that I don't have any other friends, or that people who give me things to wear are less 'special'. Quite the contrary. I'm just noting a fact: most of the things I wear weren't bought by me, they were gifts and just that. I am just commenting on how my very appearance is modified by others, some of whom no longer play.
My appearance changes with every person I meet. My attitude towards SL, among other things, changes as well. They say change, tier, and lag are the only constants in SL.
The only question is, changing into what?
(I apologise for segueing from the abstract of constant change in SL to judging age by appearance. They are connected rather weakly, so if you are lost, don't worry. Chalk it up as another Anna Hallucination.)