“How old is she?” is the correct answer. “Am, is, was, are, were, be, being, been” are example of linking verbs. Linking verbs always go before these pronouns (he, she, it, I, they, we, him, us, were, them, that, those) when it comes to asking questions.

For examples:

1.)Was she at the party?

2.)How is she doing?

3.)Where are they?

4.) Are those candies?

5.) Is this true?

For statements, the linking verbs always go after the pronouns.

Examples:

1.) She was at the party.

2.) She is doing great.

3.) They were at the playground.

4.) Those are candies.

5.) This is true.

Hope this helps clarify it for you.

PARTS OF SPEECH.