Final answer:
The youngest child leaving home for college represents a social cue marking middle adulthood's boundaries, as it symbolizes a significant change in family dynamics and social roles consistent with societal milestones.
Step-by-step explanation:
The best example of a SOCIAL cue signaling the beginning or end of middle adulthood is the youngest child leaving home for college. This event is a socially recognized transition point, often referred to as the 'empty nest' syndrome, indicating a shift in parental roles and household dynamics.
It neatly ties into the societal expectations of developmental milestones as outlined by Henig, which encompass leaving home and perhaps simultaneously suggests the parental counterpart, which is children leaving their home. These events are deeply embedded within the social fabric and are external indicators of one's stage in life.
Other options like reaching menopause or the slowing down of reaction times are more biological in nature, whereas the realization of impending retirement could be seen as both a cognitive and social cue. However, the act of children leaving home to begin their own adult lives is bound to societal practices and norms, making it a clear-cut social cue.