I wonder the same thing, but I swear DD was 2 going on 16. And that was way before the outside influences of TV and friends. I'm a tomboy...so it's gotta be genetic.
This might be totally OT, but I think in today's society kids grow up so fast and I am really unsure of why that is. Is it TV? Is it school? siblings? Music videos? Natural curiosity? Who knows really, I would just be afraid that my DD would be trying to act an age that she is not, KWIM? And the gender difference.