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. She's been to two summer camps out of state this summer with other high schoolers, some of whom were 17, and came home with a new list of friends who she has been texting a lot.
The only real disagreement we've had in regard to this is her trying to convince dh and me that it would be a fine idea for her to take a road trip with three 16 y/o boys from one of these camps next summer to go visit one of her new female summer camp friends who lives in Wisconsin -- not happening even if she was 16 or 17! When I told her that aside from the fact that these are teenaged boys and there is no way that I'd put my then 14 y/o daughter in a car with them to drive that distance, she had an answer to all of my scenarios re lack of life knowledge and how she'd know how to handle them: flat tire, lost, hit an animal, car jacking...
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. That may feed her belief that she is older than she is. OTOH, I wouldn't have had her going into 8th or 9th grade next year to head this off if it would b/c it would have been a truly poor fit for her academically and socially.
