Robert Sapolsky mentions age 3 as about the time "the brain differentially processes images within milliseconds based on minimal cues about race or gender."

Us/Them-ing looms large in primate social groups and hierarchies, with human social economic class markers sending this behavior to exponential levels.

Once upon a time I did a piece titled "I See By Your Beads..." listing some instantaneous judgments possible just by observing what beads someone was sporting.