Ack! I see I completely missed the chat session! This would have been an interesting topic to bring up. It looks like I only missed actual physics, though (darn!), so I still have a chance to discuss it.
Perhaps it's just me getting touchy about the age issue again, but I still am - and have always been - against the idea of judging users by their age. I'll admit that I've seen many posts (I'm ignoring the post that was brought up, actually) written by kids that are pretty bad. At the same time, I've seen grownups write a lot of really poor stuff, too. It's often tough to tell just who's behind the really crappy stuff, because in many cases they don't care enough to take any of this seriously - I'm talking about you, generic poster of crackpot theories of spam! - but I would assume that not all of them are kids; perhaps kids only constitute a small minority. We have no way of knowing.
I realize that despite Anna's use of "13-year-old" a lot in this post, and the example at hand, the meat of the question isn't about age: it's about expertise. Is there a correlation between the two? I would bet all my rep - and some of other people's, too! - that an increase in age means an increase in expertise, unless you subscribe to the view that science is a game of the young. I think we can all agree that that isn't the case, however; without calling anyone old, there are a lot of folks who are older than the rest but are incredibly knowledgeable. So there's a correlation. But in science, a correlation isn't enough. A causal relationship is what's needed to truly supply good evidence to a theory. And I think that there is one, at least at the level of kids.
Getting back on-topic, the reason I'm concerned is a little bit self-centered. Where do I fit in? The most I know about quantum mechanics isn't too far beyond E=hv (I know, it's nu, not v, but there's no MathJax on meta), and the most I know about general relativity is what a metric is. Same goes for a bunch of other branches of physics. I would absolutely have a _b for them. But I do know a lot more about astronomy than the average kid my age, and much of the stuff I read is college level. I'm not arrogant enough to give myself a full _c, but I'm probably not a _b, either. Chances are, I won't be asking _b-level questions. Where do I fit in? And for my overall physics average, what am I? A _b-and-a-quarter? And there are certain areas of astronomy I know more about than others. How do I describe that? Moreover, I won't be asking questions about quantum mechanics and general relativity, because of how much of a novice I am. In a couple of years, that could change, but for know, I'll stay away from those.
This - the difficulties arising from categorization - seems to be a large problem, especially at the beginner level - sort of. There are a variety of different levels of physics knowledge. There are those who don't have a good working knowledge of anything they're asking about, those who are studying stuff at school and sort of understand it - how do we differentiate among them? The same phenomenon is active in college (and, I'd warrant, graduate school) - a lot can change over four years.
I think the whole point of this is that there's no set way to lump a group of people together without getting extremely specific. Is that better than just blindly shoving them on Stack Exchange? Perhaps. But the quality and level of the questions themselves are probably a better indicator of what level a person is at - and they can show changes over time. I know considerably more than what I did 8 months ago (and mostly from outside Stack Exchange!). Would that be enough to take me a level up (in the _b, _c, etc. system)? Oh, dear, absolutely not. But it would certainly matter in terms of my participation on the site.
