Avatar is a sentient, indestructible time capsule for the entirety of human knowledge. A group of mad scientists, convinced the universe would soon be destroyed, decided to create something that would preserve a record of humanity's culture and accomplishments after the cosmic cataclysm. And they made it in the form on a cute little girl. Mad scientists are strange. Then they kicked her out into the void of space to float around aimlessly. Mad scientists are REALLY strange. Eventually, Avatar literally bumped into The Exposition and Captain Crosby allowed her to stay onboard.
Avatar is a living example of how superpowers can be inconvenient. For one thing, she's nowhere near as young as she looks. Having an indestructible body means having a body that can't grow or change in any way. Since she was built to survive in the vacuum of space, being in an environment with gravity and an atmosphere is very uncomfortable. Her body is designed to function without food, which means that if she TRIED to eat anything she becomes violently ill. Also, it can't be a coincidence that a girl who knows virtually everything there is to know about humanity tends to have a rather dim view of people in general.
And yet, despite all that, Avatar is a sweet, good natured kid (who isn't actually a kid, but you get where I'm going with this). Sure, some aspects of humanity get on her nerves (especially the Mad Scientist aspects), but all in all she's much happier being around people than being alone. After all, she spent a long time floating around alone in space, Avatar is very over the whole "being alone" thing.
A: Is there even any point in ASKING how old Avatar is supposed to be?
Q: NOPE! And not even because I don't like nailing down facts like that, but because IT'S A BIG OL' MYSTERY! We don't know when the mad scientists made Avatar, we don't know how long she was floating in space, we don't know when Captain Crosby found her, we don't know ANYTHING.
Q: Ichabod seems to know Avatar pretty well. Does HE know this stuff?
A: That's an excellent question! Maybe one day we'll find out the answer!
Q: Geez, does AVATAR even know this stuff?
A: Yup. The list of things Avatar doesn't know is very, very short.
Q: You keep saying Avatar is indestructible. How indestructible is "indestructible"?
A: REALLY indestructible. Like, the kind of indestructible that makes no logical sense. Shove You Into A Supernova And You Only Think It's Just Too Bright kind of indestructible. That's not to say that she can't be threatened by anything. We know she still feels pain, what with the whole "eating makes her sick" business, and getting stuck floating in space again would suck on general principle.
Q: Was Avatar already on board The Exposition when Ichabod showed up, or was he there when Captain Crosby found her?
A: Another excellent question! Maybe one day I'll figure out which it is! (That's not me being a sassy brat like earlier, I'm honestly not sure right now. No doubt some future story will address this issue, and when I write it, I want my main concern to be "what would be the best story?" not "How well does this conform to the stuff I said a couple years ago?")
Q: So, how grown up is Avatar supposed to be? Like, not in years, but mentally. Is she supposed to be an adult in a child's body, or is she just a kid who's been a kid for a looooooooong time?
A: Um... Yes? The snarky answer is "whatever would be funniest in this specific page", but it honestly is a bit of both. With all her nasty experiences so far, plus having ALL OF THE KNOWLEDGE, she's obviously more mature than someone who actually is the age she looks. And yet, she also ISN'T that mature. I mean, she's never gone to school or held down a job or faced the terrifying prospect of aging or what have you. Heck, she's never even experienced puberty. So, yeah. Somewhere in the middle. Write your Fan Fics accordingly.
Q: If eating food makes Avatar sick, why doesn't she get sick from consuming the various particles and crap in the air?
A: Um... because I wasn't thinking that hard when I came up with all this stuff? Actually, I'd already said being inside an atmosphere is uncomfortable for her, just because of the air pressure and what have you. Let's just say that it's also because she's taking in a little bit of air every time she talks or something. Acceptable?