I was asked to do this when I logged in tonight, and thought, WTF??? My birthday is the same as it's always been. Turns out some people(not necessarily here) never entered their birthdays when they created their accounts and they need to because of some Proboards reason.
Anyway, if you were asked to update it and were wary, it's cool. It's just paperwork and your birthday still won't show unless you want it to under your privacy settings in your profile.