That is incorrect. We are not blank slates. Our brains vary in wiring.
I would argue against this statement because its our environment and the way we are raised (not to mention presented with certain options), along with exposure to certain information (and a whole bunch of other factors that have nothing to do with 'mental wiring') what varies quite a lot.
Even people who were born in the same environment, exposed to same 'education' will be each treated differently, wether by their parents or their peers, and their exposure to information (relevant or otherwise along with education will be different) - in some cases, the differences will probably be subtle (in others, large), but it would still have a profound impact on ones development (ranging all the way back to fetal development - nutritional intake/diet, external stimulation impacting the body and the fetus growth - all of those are factors that can and probably do influence Human development).