["Behavioral Sciences &the Law, EarlyView. ", "\nABSTRACT\nYouth justice systems are frequently justified by reference to developmental change, yet chronological age is often treated as a proxy for underlying psychological processes. This paper develops a Developmental Retribution and Reciprocity Model (RRM), integrating evolutionary criminology with contemporary developmental neuroscience to clarify how reciprocity, retribution, and cooperation are expressed across development. Behavioural economic tasks and hypothetical crime scenarios from an adult sample (N = 300), spanning late adolescence and early adulthood, are used to examine whether age is associated with positive reciprocity, negative reciprocity, retaliatory punishment, and offending‐related tendencies. Developmental theory would predict age‐related differences across this period; however, no meaningful associations with age (from the age of 16 onward) were observed. The absence of age‐graded variation in reciprocity and retributive responding within adulthood is consistent with the possibility that the underlying motivational structures captured by RRM stabilise earlier in development, whilst their behavioural expression continues to be shaped by regulation and context. The paper therefore emphasises the importance of focussing youth justice interventions and future research on these earlier developmental periods, where socialisation, legitimacy, and cooperative norms may be most effectively shaped.\n"]