Personally I just put in a date of birth that fell in the same year as my real date of birth. That way people know more or less how old I am, which is all that matters to me. There are too many phishers and identity thieves out there nowadays ready to collect and collate personal information.
I see some people make identity theft easy by giving away their full name, date of birth, address and sometimes telephone number. Why not just add their social security number and bank account details too, save the phishers a few minutes work?