It would depend; you could start when they have their first interpersonal disappointment.
You fail to realize the importants of putting your kids in an ideal world. It creates expectations. If you teach kids that they will never be an astronant, and should instead be a chemist, they will become a janitor,
Everyone over inflates there vision of the future it's healthy, if you don't belive that your future won't be much better than the present you settle with the now and become apathetic and unmotivated(see black people).
Everyone falls short of their dreams regardless of what they are, if you dream to be a janitor you goto prison etc.
It's just how were wired from evolution.
Also if kids are not tricked into believing that there's a right and wrong in this world, they become valuesless and aren't able to bring order to their lives.