If it's a properly fitting bra you should hook it on the first hook. The second and third are supposed to be used for when the bra stretches, because the elastic will naturally stretch with time. So when they're new I want them to be on the first hook.
It depends on the brand, shape, fit, and comfort level. I have a pretty big bust that doesn't stop growing, and im 25!!! So i find a bra that will fit comfortably with the shape i like that will clasp on the tightest and as i start to grow out of the bra i move the clasp each time, i also buy bras with 4 clasps going down, and 4 clasp options. But like i said it all depends on comfort level!!!