A friend of the family had their daught go through the reduction surgery WHILE she was in high school. I never noticed how big she was, but I suppose if they're in the way and causing problems such as back pain, then it would be benificial to go ahead and do it.
I've always thought about getting implants myself because I'm not terribly big. However, I know my mother was flatter than an ironing board and once she started having children they got a lot bigger.
My ex's sister on the other hand had pretty decent sized breasts I guess before she had Grace and then afterwards she lost a lot in size. So I guess there's no written rule