When they hit 30s ladies usually get a tad more sensitive when asked about their age. More so if they are unmarried. Thus such numerical data become a delicate isue.
However, you can ask for her email address which more often than not includes her birth year. Women tend to use their real name when opening an online account so the user ID are usually unavailable unless added with some numbers. That numbers usually ends up being their birth year.
It's kinda silly, but that what I observed almost on a daily basis.