In spanish, what age are you when you go from a "tu" to an "usted"??

Age and amount of respect DO go hand in hand however the use of tu or usted is more a respect thing.

No certain age. But usually someone of higher status than you. if you are 20 you would say usted to a 30 year old or something. With people you dont know you use usted. With close friends you can use tu. You should know when to use it.

It's not an age thing, it's about formality. Think of it like this: in English, if you would address someone as Mr, Miss, or Mrs., then they should probably be called usted in Spanish. If you're on a first name basis with them, call them tu.

It doesn't matter the age, it matters what their ranking is. If they are a teacher, boss, or adult, you'd refer to them as "usted". If it was a peer, friend, dog, or sibling, you'd call them "tù". Hope I helped.

ud, is for respect. So your grandparents, teachers, etc, Maybe your parents, but not likely.

use tu for the others.

there is no age its just a matter of respect its like compared to sayn "yo wuddup cuz" to "hello how are you sir"