The present three-wave panel study (N = 968, (Formula presented.) = 11.30, SD = 1.06) examines how developmental factors—pubertal timing and heterosocial involvement (i.e., one’s involvement in cross-sex activities)—influence early adolescents’ level of media internalization. We hypothesized that early pubertal timing positively moderates the association between sexualizing magazine reading and media internalization. Next, we argued that increased heterosocial involvement will weaken the amplifying influence of early pubertal timing on the aforementioned relationship. Both hypotheses were confirmed. For early adolescents who mature earlier than same-age/sex peers, reading sexualizing magazines resulted in more media internalization. Furthermore, our results showed that moderate to high cross-sex peer interactions can serve as a protective force against the negative influence of early pubertal timing. These results highlight the influential role of appearance-related developmental factors in the processing of sexualizing magazine content and point to the potential protective role of cross-sex peer interactions in media internalization.