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

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

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.