When does the brain stem appear in humans?
I'm not sure this question is answerable at that specificity. There isn't a discrete point in time where the cells are definitively a brain stem rather than a precursor structure such the bulge mentioned above. One could identify a set of a cells in earlier structures that eventually are located in the adult area we call the brain stem, but that doesn't mean at the point those cells are identifiable that they are IN the brain stem during development.