My dd13 is this way as well. She isn't defiant, she just feels older than she is and wishes that she could be at times. I've wondered if some of this is related to grade placement (she'll be a high school sophomore in the fall), but I think that a lot of it is just who she is.
I recall a conversation we had a bit back in which I was reminding her that she is 13 and she told me that she wasn't really 13, she was just chronologically 13, but more like 16 or so in reality.
Yup! I've heard that line, too!
It isn't a huge push from her in that I doubt that she really thought that we'd agree to something like that, but it is more that she feels a lot older than she is and wants to freedom to go do things that sound fun like that.
I think that another aspect that we've had to consider is that b/c she is placed with older kids most of the time and it is a better social fit for her, these other kids either assume that she is older than she is or treat her as an age peer b/c she is an emotional peer even if they know that she is younger.
I agree-- my daughter is
not particularly unhappy to be only 13, but she occasionally chafes at the asynchrony which is her reality. Socially, she is also aware that to 'pass' with her peer group, she often has to not give her chronological age away too soon. This can lead to her
acting older in some settings, and to making some pretty amusing assumptions about what we should permit her to do... though like Cricket's DD, I'm also pretty sure that she doesn't
actually think that we should allow her to go camping for a week with her 17yo (mixed gender) friends, etc. The things that she
does think we have "no right" to tell her not to do (Skype, etc.) she simply ignores us about and does on the sly. That's got it's own problem, but I'm not convinced that it's that she really WANTS to be older than she is or more autonomous, other than in those areas of... er...
disagreement. 
Some of that is being a teen of any cognitive ability or personality, and it's too hard for me to tease apart which is which.
I just know that she doesn't seem to have the kind of existential angst about her life that I did at her age. My life as a child was
not at all consistent with my inner compass, and I felt powerless to live according to my own needs and values. I think that DD doesn't have those feelings.
