Social knowledge refers to the ability to analyze and reason about social situations in relation to social rules which are essential to the development of social skills and social behavior. The present research aimed to assess these abilities with the “Social resolution task” in a neurotypical population of 351 children (4 to 12 years) and 39 young adults, and in 20 participants (10 to 18 years) with Down syndrome. Results showed that young children aged 4 to 6 were well able to distinguish between appropriate and inappropriate social behavior but they had significantly more difficulties in judging and identifying social cues for the transgression of conventional rules than for moral ones. Between age 4 and 8, their social reasoning was mainly based on factual answers, while older children showed significantly more social awareness, making more reference to emotional and social consequences for the “victims”. The representation of a more universal applicability of social rules seemed to develop later in childhood, as of age 8. In contrast, participants with Down syndrome exhibited significantly more difficulties in judging, identifying and reasoning about transgression of social rules without social awareness. In conclusion, the results have shown that social reasoning abilities develop throughout childhood. Social awareness seems to have a long developmental course, which includes a sensibility about welfare and intersubjectivity, critical for the development of prosocial behavior. The clinical population with difficulties in social interaction and socio-emotional behavior could benefit from an early assessment and from learning social reasoning abilities to improve social skills.