To talk about such general actions, it is common to use pasiva refleja with the se pronoun, instead of "tú":
- Cuando se habla con una persona mayor, se usa usted en vez de tú.
If you want to use tú (or usted), you need to use the imperative form:
- Cuando hables con una persona mayor, usa usted en vez de tú.
If you want to use the verb dirigirse, you cannot easily use the pasiva refleja, because dirigirse is a pronominal verb that already contains a se. In this case, you either use the imperative form:
- Cuando te dirijas a una persona mayor, usa usted en vez de tú.
or use uno as the subject, if you want to make it more general:
- Cuando uno se dirige a una persona mayor, usa usted en vez de tú.
The form usares is future subjunctive. This tense is almost never used anymore, so you can safely forget about it. It only appears in some legal texts, referring to events that may happen in the future. For example "A person who causes harm to another without provocation shall be guilty of assault": the word causes would be in future subjunctive. In almost all non-legal contexts, the present subjunctive is used for this purpose instead.