What stage of life she's at? But she (like Hanners) is a clear case of partially-arrested development!
Socially, she's probably close to scarred's 11 1/2.
Emotionally, a little older (teenage, still, but I think she's been held back some by social rejection).
Intellectually? Who knows - haven't seen enough of her to guess. Most of the verbal sparring the other characters get into denotes a fairly mature intellect (the ability to run with a gag is not something most kids can do).
Physical age? She's obviously old enough to drink in a bar, but believe me, that doesn't mean much!
The more we get to know her, the younger she seems to be acting. Which is probably the confusing factor that led to the creation of this thread!
God, I need to get a life and quit analyzing webcomic characters. It's a really sad hobby...