How would most spanish speakers ask an elderly person how he/she is doing?

The correct answer is: ¿Cómo está?  or ¿Cómo está usted?  that means How are you?

Explanation:

Most Spanish speakers address formally to elder people, as a sign of education or politeness, so even if they are referring technically to the singular second person and they could use correctly the pronoun "tú" (you), the pronoun they use is "usted" that follows the rules of the singular third person (she/he). So, when asking how someone is doing in the singular second person would be: ¿Cómo estás?; and in the third person would be ¿Cómo está?

Note that in Spanish is not necessary to include the pronoun in the question, for the conjugated verb refers directly to the subject, but it could be added in terms of emphasis.

So, asking an elder how is doing, would be grammatically as if we would be referring to a third party and therefore the correct answer is: ¿Cómo está?

Now, if we want to emphasize the subject we are referring then it is also correct to ask adding the word "Usted" at the end of the sentence, like this: ¿Cómo está usted?