I live in Montevideo, here people use "usted" (the formal "you" in Spanish) to show respect to older people. But there's no strict rule about when to switch from "tu" (informal you) to "usted". So you kind of guess based on how old someone looks.

The problem is, if you say "usted" to someone who doesn't see themselves as old, they might take offence. So, trying to be polite in Uruguay can backfire if your mental age calculation is off!