It is quite proper in American English and easily understood in context. I suppose it may not standard for this course but it is no more of a stretch than some of the other phrases that are accepted. I admire your diplomacy in matters such as this and thank you for answering my question. English has a habit of making questions of declarative sentences by using emphasis and tone. Just wondering how often this occurs in Polish.
In Polish, yes/no questions are usually created by just putting a question mark at the end, although you can start them with the word "czy". So basically emphasis and tone are enough.
The thing we need to remember when considering English answers is that there are many people that learn English here. It's an official Duolingo recommendation to take the 'reverse course', so for example for Polish people who want to learn English to not only take English for Polish course, but also this one. And now, if a Polish student writes "She is how old?" on an English test... well, I don't think that would be accepted.
That is an interesting point. Over the years of trying to learn Polish with this course, I have wondered how much of the difficulty of answering questions is due to the large number of variables of students such as standard American and British English as well as regional differences. Now when you throw in Polish to English students it makes sense why sometimes translations are accepted and others not. Thank you for the explanation.