You don't, you have the option to say "Neither", or "Prefer not to say"  for your gender and we are happy with that. Similarly you can make something up or ignore the age box. We use this data to get a picture of what the National Cipher Challenge community looks like and don't store it with your registration info. We recognise that "Neither" could look a bit dismissive and we did consider a range of other options, but in practice we don't need to know so probably shouldn't be asking, though that might change in the future.