the second bit is more formal, " You're six years old, aren't you?"
because, the other sounds very demanding or imperative, like as if you have to answer the question. so, it has to be the second line.
Is this for a book or a play
Explanation: A writer should always include a works cited page because you don’t want to be called out on plagiarizing. If you have quotes or information from a website, book, article, or any source you used then you need to give credit to that person/people because you didn’t come up with the information yourself.