Xbox live help

[adam3112]

Hi i'm trying to get set up to xbox live and because i'm 17 the thing says i need parental supervision for aload of stuff but when i first got my 360 i don't rememeber entering my date of birth for anything and i was aware that you needed to be 18 to be able to get full access over xbox live. So how does it know my date of birth and how can i change it so it thinks i'm older! plz help thnx!