Señor

As you know, señor, señora and señorita equate with Mr., Mrs., and Miss, respectively. However, the Spanish terms may be used with first names by service employees. A doorman might say: Señor Carlos no está. Mr. Carlos is not in right now.

Joven

More generally, these terms of address can be used when someone's name is unknown or is unimportant. If a young woman dropped something on the street, a stranger might call out señorita after her to get her attention. A young man, or even a not-so-young one, would be addressed as joven. Señorito, little gentleman, is out of fashion.

Se ve señor

Se ve muy señor, he looks old (for you). I once overheard: No, era un señor. No, the customer (that I am telling you about) was a grown man around age 40, not a young man like yourself.

Sí, señor

For comical effect, the emphatic replies of sí, señor and no, señor can be used even regardless of who is being addressed. I once heard a son reply no, señor to his mother. The idea was that of no, sirree, no way. I have also witnessed a guy who said no, señora to his girlfriend, meaning, no ma'am.