The current study examined the development of bullying and defending over a 1-year period as related to friends’ influence and individual and friends’ moral disengagement (i.e., self-justification mechanisms that allow one to avoid moral self-censure of transgressive actions) in children and young adolescents. Via longitudinal social network analysis (RSiena), it was tested whether similarity between friends in bullying and defending developed over time due to friends’ influence, while controlling for friendship selection processes, and whether there were differences in these processes between children (age 9–10 years; n = 133; 42.9% girls) and young adolescents (age 11–14 years; n = 236; 40.6% girls). Results showed that individuals selected peers as friends who were similar in bullying and became more similar to friends in bullying over time, but only in early adolescence. Moreover, there was marginal support that friends’ influence was stronger in young adolescents with higher moral disengagement. In early adolescence, bullying was also indirectly influenced through friends’ moral disengagement, with different effects for boys and girls. With regard to defending, young adolescents maintained friendships with peers who were similar in defending, and became more similar to friends in terms of defending over time. These findings suggest important differences between late childhood and early adolescence in socialization processes and indicate that in early adolescence, friends’ influence on the development of bullying is partially affected by moral disengagement.