Because it is not a question. If you want to ask a question you should use the axuilary verb, is here, in the beginnig of the sentence. , axuliary verb comes after them.
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Aiemen1) How old is she? - This question is correct because it is a direct question.
2) How old she is? - this question is not correct. To make it correct, you should add some words to the question. For example, I want to know how old she is. (Note