You should definitely ask a woman her age. Not only that, but you should also ask to see some I.D. and then make photocopies of it for your records. Women love a man who cares about attention to detail
Seriously though if you're wondering how old a girl is who looks sort of between 16-25 surely you can get a good idea just by asking what she does for a living. If she goes to school there's your answer, if she works then you ask how long she's been there..."oh right, was that your first job after school then?". I suppose that avoids you having to outright ask their age. Even still, I wouldn't have thought most women would mind being asked their age - they just like the attention. I think the whole "never ask a woman her age" thing is from a bygone era.