There seems to be an error in the description of the brass dragon NPC Glisten. The description says she enjoys taking human form to attend social events, however the stat block preceeding her gives her no power that allows her to take human form.
To the best of my knowledge a young brass dragon wouldn't be able to take human form anyway. (Not by spell or special ability anyway...)
I guess Glisten was supposed to be a bronze dragon (who can take alternate form at the "young" age category) but became a brass dragon when someone wasn't looking