How do you feel about scheduled weaning?

I have no knowlege or experience with human babies, so that may be another issue. But horses and puppies – is it more psychological painful to do it after a few weeks or months, or where practical? Why not let the little ones stay with Mom until circumstances intervene to make separation necessary? Seems it would be easier once the foal or puppy had develeped themselves further, beyond childhood, but I am no expert. Could someone enlighten me with facts or opinions? Thank you?