I suspect that whoever's writing this isn't really a fifteen-year-old girl, any more than I think Christopher Walken is really writing this. In the case of the former, I'm thinking it's the Holden Caulfield syndrome, where it takes a good distance on the teenage years to create a character who walks and talks the way we all wish we had at that age. And I also think that because 100% of the people I knew at fifteen couldn't write for shit. In the case of the latter, I don't know why he'd bother to blog, but then again, there's no reason he wouldn't.
I could be wrong. Either way, they're super much fun to read.