How do I hide Birthday date display on Hi5 social site?

I am not sure where to ask this question but because it is about security over internet ie. about my privacy. I friend invited me to join Hi5 site which I suppose is something like orkut. I Joined it but now there is no option I can find there to hide my birthdate from public. I only want that my friends can see it but there is no option to hide it from general public. In it I can see option for privacy but just birthdate option is not there. Please help me I also searched internet but there was no help there. Is it not there to hide ?