Why is it that guys who are 16 to 17 years old are called sir Mr and Monsieur?

Because they are too young for treating them as children. Many of them you even can't guess they are 17 because look like 22. On other hand many people look younger than they exactly are. To not make faux pas it's better to call Mr. I think it's quite awkward to use "you" in front of somebody who looks as 17 but in fact he is 25...