Well, according to my American sources it is perfectly reasonable to say: "What is her age?" and thus it is chosen as the best translation because it is also the most literal translation. I have added "What age is she?" since this would also be correct. "How old is she?" is not that similar, and thus I have not added it this time around, but if you really feel that it is the same as the two others, then I'm open for suggestions.
I was talking with my friends (I'm American) and we do say "what is her/his age?" but almost always specifically when referring to babies/children and pets. So we would never talk about a grown woman or man and say's what's their age, but rather how old is he/she. In think, in the end, we're splitting hairs and go with what feels best. I think we all learned something from this and that's the point.
Just a note that the 's is NOT accepted as shorthand for is so beware if you're like me and refuse to complete a lesson without 100% accuracy and feel compelled to quit the lesson after a mistake and do a practice in order to have a perfect track record. By the way, some of the practice exercises are being pulled from the depths of the tree I reached several years ago and not recently so that's incredibly infuriating.