Hi Linda! It's not about being young or old :) but about how much you know that person. To your mother, father, brothers or friends you should say "¿Cómo estás (tú)?". But when you talk to your teacher, to your doctor or to someone you don't know in the street to ask for directions, it'd be more advisable to use "¿Cómo está (usted)?" It's way more respectful. Hope that helped!!

As Alejandra said, it is more respectful to use that formal way (usted) when you are talking to a person you don't know very much. But you are right when you talk about ages, because we always treat the informal way people that is our same age.