My mum had it done well after her childbearing days and it was none too soon. I also know someone who had it done before kids and breastfed her first one without knowing it wasn't supposed to work! When her second came along she had a bit more trouble and was told then that it wasn't supposed to work the first time. She's BFing her third baby part time, cos he was a hungry little tacker who required more than her few functioning ducts could manage, and it was the first time she could notice the surgery having an impact.
There are conversations you can have with the surgeon about needing to BF, it's just that a lot of the time BF doesn't even rate for the young women who are desperate to get it done (reflects our society, cos these women, in hindsight, would have taken more care in their decisions) and it's not entirely the surgeon's fault that provision for BFing wasn't made.
The super important thing to remember is that you MUST go to a plastic surgeon, not a cosmetic surgeon. A plastic surgeon is a proper doctor and a cosmetic surgeon could be anyone, a GP who did a brief course even. There are nightmare stories from women who didn't know there was a difference