Question

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

Answer 1

You would say "usted" but in a sentence it would be, "Como esta usted?" 

Remember "Usted" is used to people older than you as in teachers, parents or retail workers who are the same age as you.

Answer 2

Say "hola, como estas?" or if they ask you first reply with "muy bien, y usted?" never reply to an elder with "y tu?" most of them see it as disrespectful