I was always a big believer in not until high school but I caved and got DD2 one last Christmas, she was just 10. She is on a prepaid and has been very sensible with it, its mainly for when she is out with friends and having sleepovers etc.
We bought it from Aus post shop and it cost $45 with $50 of prepaid AND a bottle of Brittney Spears perfume which she gave to her older sister.