We don’t have any authentic hadith in our books that states she was 9. The hadith you mentioned is the only hadith in our books, but it is not attributed to our Imams. It is from Ismail. Hence, it is not Hujjah (proof) for us twelver Shias. Assuming Ismail said that, he either was saying what the majority would state or he said it out of Taqiyyah (or he gave his own opinion). In any case, we do not have a hadith attributed to an infallible Imams that says that.