This would be very useful for demographic analysis. On one of our sites we collect “Age Band” by decade for this purpose, which is a bit silly as it degrades by 10% in accuracy per year! We probably should have collected year (or decade) of birth instead but hey, hindsight is 20/20.
It would be nice if DOB was baked into an official plugin like Cakeday, as long as the field was reachable by Polls Breakdown. As a configurable option of course in order to cater for those forums where privacy is rated more highly.