I think a date picker just make the move more complicated to enter one's birth date.
As already mentioned, a combination of drop lists for days and months with a text box for the year seems the most efficient for a user. It takes just less than 10 seconds to enter a birth date this way, which is far quicker than a date picker: thanks to the tab key (which all users should learn to use to complete a form), it's fast to go from one form element to the next one.
At least under Macintosh (I don't know about Windows), you also can use the keyboard to access date inside select boxes: thanks to the tab key, you get the focus onto the form element, then press the arrow key to drop down the list, then type for instance 1967 and you get there in the blink of an eye…