Understanding when children develop a sense of group boundaries has implications for conflict and its resolution. Integrating social identity development theory and the developmental peace-building model, we investigated whether preferences for ethno-religious ingroup symbols mediate the link from child age to outgroup prosocial giving among 5- to 11-year-old children from both majority and minority backgrounds in three settings of protracted intergroup conflict (N = 713, M = 7.97, SD = 1.52, 52.6% female). Participants represented the conflict rival ethno-religious groups in each setting (Northern Ireland [n = 299]: 48.5% Protestant, 51.5% Catholic; Kosovo [n = 220]: 54.1% Albanian, 45.9% Serbian; Republic of North Macedonia [RNM; n = 194]: 45.9% Macedonian, 54.1% Albanian) and were largely from lower- to middle-class families; 4% of participants from other ethnic backgrounds were excluded from the current analyses. Multiple-group, bias-corrected bootstrapped mediation found that ingroup symbol preference mediated the link from child age to outgroup prosocial giving; that is, older children expressed higher ingroup symbol preference, which was linked with lower outgroup giving. Across Northern Ireland, Kosovo, and the RNM, there was some significant variation in the strength of specific paths; however, there was a significant indirect effect in all three settings. The findings advance cross-cultural understanding of how age relates to ingroup symbol preferences and outgroup prosocial giving across the elementary school years, with implications for children's long-term peace-building contributions in three conflict-affected societies.