Birthdays and Getting Old

Girl:  Oh wow.  I just realised, when I see you next time, I'll be old!  It's my birthday next week.
Me: Ooh, what day?
Girl: Wednesday.  I'm turning 15, so I'll be old.
Me: Okay... [We talked a bit about birthday activities]
Girl: We've got so many tests next week, but none on my birthday -
Me: That was nice of them.
Girl: But there are four the next day.  Oh, and mum had me sign this Medicare thing -
Me: Wow, you are getting old!
Girl: - and I just signed it -
Me: Ooh, you should always check what you're about to sign.
Girl: I know!  But mum just went, like, agh! [high pitched 'mother' voice] You should always read it first before you sign! [normally] But I'm, like, you're my mum, I should be able to trust you!
Me: One would hope so, anyway.  So, why is 15 classified as 'old' now?
Girl: Well, because I'm born in November, I'm older than most of the girls in my grade, so they're all teasing me, saying 15 is half-way to 30... which it is... So I'll be old soon... [sad face]