1 What are linking verbs?
Linking verbs are the words that give information about the subject, such as a relationship or a condition. In other words, these are the verbs connecting the subject to the predicate noun or adjective. Some examples of linking verbs include am, are, were, is, being, been, was, etc.
2 Is it correct- How old she is?
Yes, there is nothing wrong in this sentence- How old she is, however, only if it is without a question mark, as this sentence implies that the person is making a statement. If a person is asking a question, then this sentence is not correct. Although, it is correct to say- Can you guess how old she is?
3 What will be the correct sentence if I want to ask a question about the age of the third-person female?
If you want to ask a question you can say- how old is she? Then the other person will let you know how old she is.