Some preface: The Worldstone of Sanctuary was changed after the hybrid of angels and demons (the nephalem) was seen to be too powerful. The stone's new goal was to reduce the power of the nephalem so they would not be as great of a threat. However, after the events of Diablo 2, the Worldstone was destroyed and allowed nephalem to come about again.
Now what confuses me is the timeframe of the whole ordeal. In the Drowned Temple in Diablo 3, players can find a lore book that has a nephalem that says "The Worldstone has been changed. Our children are born weak and suffer short lives.", implying that the effect of the Worldstone does not come into effect immediately but rather, affects the newer generations. In other words, the first nephalem were unaffected (Kalmor is an example of this) but their children suffered. Over time this lead to a lack of nephalem and the oncoming of humans.
Since the effects of the Worldstone appear to be generational, how are characters obviously born before its destruction, like the male barbarian (older and experienced/lived on Mt. Arreat prior to big boom) or witch doctor (old), nephalem? Shouldn't nephalem only begin to appear in the generation born after the Worldstone's destruction?