"Are you nine years old?"
Translation:¿Tú tienes nueve años?
Your answer "Tienes nueve años" would be perfect. As I said before, it would be for me even better than the given answer but for any reason it is not accepted yet. I hope that it would be corrected soon. In Spanish we often omit the subject of the sentence unless it gives essential information, and that is not the case.
I know that you didn't use the "ñ" because you don't have the spanish keyboard but just for you to know "año" is "year" and "ano" is "anus". If you use "n" instead of "ñ" maybe one day you may have a funny misunderstanding :) :)